@Swaiba: You are right,I should have added bit more details to my first post so you better understand what I did do.
This version has not been modified beside that fact that I do have a customized theme.
I haven't tried to login since nearly 2 years but when I did try yesterday I apparently always typed in the wrong password.
So I search a bit on how to recover the password in Silverstripe and did find this and this posts in the forum.
I then did try all steps that have been described there but non of them worked.
So I went to myphpadmin and run the statement
DELETE FROM Member;
As also this didn't help I did some manuall changes in table Member and MemberPassword where I set email to admin and added a not encrypted password as I thought this will enable me again to login after calling test.mydomain.de/?flush=all .
Of course it didn't work and I have no idea how to fix this now?
What would you recommend?
Im using FileZilla and if you can tell me which details you need I will definitely post them here as well.
Thanks so much for your help!