# include<iostream>
#include<string>
#include<fstream>
#include <stdlib.h>
#include <sstream>

using namespace std;

int anzahlStudenten = 0;

struct Student
{
string firstname;
string lastname;
char sex;
int matriculation_number = -1;
double final_mark = -1;

Student *prev = NULL;
Student *next = NULL;
int wert;
};

//Student students[maxAnzahlStudenten];

void importStudenten();
void exportStudenten();

Student *head = NULL;
Student *tail = NULL;

void print() {

Student *current = head;

if (head == NULL)
cout << "Liste leer..." << endl;

while (current != NULL)
{
cout << current->wert << " " << current->firstname << " " << current->lastname << " " << current->sex << " " << current->matriculation_number << " " << current->final_mark << endl;

current = current->next;
}
}

void insert_Student(Student *newStudent, string vorname, string nachname, char geschlecht, double matrNr, double note)
{
// Fall 1, die Liste bekommt ihr erstes Element
if (head == NULL)
{
head = newStudent;
tail = newStudent;
}
// Fall 2, die Liste bekommt weitere Elemente
else
{
newStudent->next = head;
head->prev = newStudent;
head = newStudent;
}

newStudent->firstname = vorname;

newStudent->lastname = nachname;

newStudent->sex = geschlecht;

newStudent->matriculation_number = matrNr;

newStudent->final_mark = note;

cout << "Student wurde angelegt." << endl;

anzahlStudenten++;
newStudent->wert = anzahlStudenten;
}

void removeStudent(int searchvalue)
{
Student *current = head;

while (current!=0)
{

if (current->matriculation_number== searchvalue)
{

if (current == head)
{

if (current == tail)
{
head = NULL;
tail = NULL;
}

head = head->next;
head->prev = NULL;
return;
}

else if (current == tail)
{
if (current == head)

{
head = NULL;
tail = NULL;

}
else
{

tail = tail->prev;
tail->next = NULL;

return;
}

current->prev->next = current->next;
current->next->prev = current->prev;
current->next = NULL;
current->prev = NULL;

}

cout << "Student gelöscht";
break;

}

current = current->next;

}

}

/void removeStudent()
{
// Wenn Liste nicht leer
if (head != NULL)
{
// Wenn Liste ein Element hat
if (head == tail)
{
head = NULL;
tail = NULL;
}
// Wenn Liste mehr wie ein Element hat
else
{
head = head->next;
head->prev = NULL;
}
}
}/

void importStudenten() {
cout << "Sie haben Menuepunkt 5 gewaehlt" << endl;
string vorname = "";
string nachname = "";
char geschlecht = '0';
double MatrNr = 0;
double note = 0;

ifstream csvread;
csvread.open("data.csv", ios::in);

if (csvread)
{
string s = "";
int counter_field = 0;
int counter_student = 0;

while (getline(csvread, s, ';'))
{

if (counter_field == 0)
vorname = s;

if (counter_field == 1)
nachname = s;

if (counter_field == 2)
geschlecht = s.at(0);

if (counter_field == 3)
MatrNr = atoi(s.c_str());

if (counter_field == 4)
note = atof(s.c_str());

counter_field++;

if (counter_field > 4)
{
insert_Student(new Student, vorname, nachname, geschlecht, MatrNr, note);
counter_field = 0;
counter_student++;
}
}

csvread.close();
}
else
{
cout << "Fehler beim Lesen!" << endl;
}
}

void exportStudenten()
{

Student *current = head;

cout << "Sie haben Menuepunkt 6 gewaehlt";

// exportieren
ofstream csvwrite;
//ofstream streamforoutput;
csvwrite.open("data.csv", ios::out);

while (current != NULL)
{
if (current->matriculation_number != -1)
{
csvwrite << current->firstname << ";";
csvwrite << current->lastname << ";";
csvwrite << current->sex << ";";
csvwrite << current->matriculation_number << ";";
csvwrite << current->final_mark << ";";
}

current = current->next;
}

csvwrite.close();
cout << "datei wurde exportiert" << endl;

//break;

}

void linearesuche(int searchvalue)
{

Student *current = head;

while (current != NULL)

{

if (current->matriculation_number == searchvalue)
{

cout << current->firstname << " " << current->lastname << endl;
return;

}

current = current->next;

}

}

void bearbeiten (int searchvalue)
{

Student *current = head;

while (current != NULL)

{

if (current->matriculation_number == searchvalue)
{
cout << current->firstname << " " << current->lastname << " "<<current ->sex<<endl;

cout << "1.Geben sie'1'ein um den studenten zu loeschen " << endl;
cout << "2.Geben sie '2' ein um den studenten zu bearbeiten " << endl;
int input = 0;
cin >> input;

if (input == 2)
{
int wert;
int a = 1;
int b = 2;
cout << "geben sie a um studennten zu beahlten und b um es zu verändern" << endl;
cin >> wert;
cout << "Neue Daten eineben" << endl;

cout << "Firstname :";
cin >> current->firstname;
cout << "lastname:";
cin >> current->lastname;
cout << "sex:";
cin >> current->sex;
/cout << "mariculation number:";
cin >> current->matriculation_number;/
cout << "Note:";
cin >> current->final_mark;
}

if (input == 1)
{
cout << "input 1";
removeStudent(searchvalue);

}

}

current = current->next;

}

}

int main()
{

/students[0].firstname = "Daniel";
students[0].lastname = "Kreis";
students[0].sex = 'm';
students[0].matriculation_number = 12346;
students[0].final_mark = 1.3;/

while (true)

{
int choice;
string vorname = "";
string nachname = "";
char geschlecht = '0';
double MatrNr = 0.0;
double note = 0.0;
int searchvalue = 0;

std::cout << "Studentenverwaltung V0.1" << endl;
cout << "" << endl << endl;
cout << "1)Student/in auflisten" << endl;
cout << "" << endl << endl;
cout << "2)Studen/in anlegen" << endl;
cout << "" << endl << endl;
cout << "3)Student/in suchen" << endl;
cout << "" << endl << endl;
cout << "4)Student/in bearbeiten" << endl;
cout << "" << endl << endl;
cout << "5)Studierende importieren" << endl;
cout << "" << endl << endl;
cout << "6)Studierende exportieren" << endl;
cout << "________________________________" << endl << endl;
cout << "7)Programmende" << endl;

cout << "Wählen Sie ein Menuepunkt aus: ";
cin >> choice;

switch (choice)
{
case 1:
cout << "sie heben menüpunkt 1 gwählt" << endl;
print();
break;
case 2:
cout << "sie haben Menüpunkt 2 gewählt" << endl;
cout << "Vorname: ";
cin >> vorname;
cout << "Nachname: ";
cin >> nachname;
cout << "Geschlecht: ";
cin >> geschlecht;
cout << "Matrikelnr: ";
cin >> MatrNr;
cout << "Abschluss Note: ";
cin >> note;

insert_Student(new Student, vorname, nachname, geschlecht, MatrNr, note);
break;
case 3:
cout << "Sie haben Menuepunkt 3 gewaehlt" << endl;

cout << "schreiben sie die Matrikelnummer des gesuchten Studenten" << endl;
cin >> searchvalue;
linearesuche(searchvalue);
break;
case 4:
cout << "Sie haben Menuepunkt 4 gewaehlt" << endl;
cout << "schreiben sie die Matrikelnummer des gesuchten Studenten" << endl;
cin >> searchvalue;
bearbeiten(searchvalue);
break;
case 5: importStudenten();
break;
case 6: exportStudenten();
break;
case 7:
cout << "Das Programm wird beendet" << endl;
return 0;
break;

default:
cout << "Das Programm ist zum Ende gekommen" << endl;
break;

}

} while (true);

cout << endl;
system("pause");
return 0;
}

Dann: ich sehe überhaupt nicht, dass du irgendeinen Studenten löscht. Es befindet sich kein "delete" im Code.

Und dann schau mal in das removeStudent:

if (current == head) {

                if (current == tail) {
                    head = NULL;
                    tail = NULL;
                }

                head = head->next;
                head->prev = NULL;
                return;
            }

Welchen Wert hat head->next , wenn vorher current == tail war? (Hint: nicht machen!)

Achso, und da das ja wohl C++ sein soll:
- verwende nullptr statt NULL
- verwende Smartpointer (oder denke an das delete beim entfernen)
- gib deinem insert_Student nur den Studenten als Parameter, nicht noch irgendwelche Initialisierungen für Student selbst, die besser im Student-Konstruktor aufgehoben wären

bestimmt noch viel mehr...