thank you both for your help! and sorry for my late response.
I don't run mods on my server, so this should not be the problem.
I deleted the Eco folder on Thursday but nothing changed after I tried to restart the server.
However today I managed to run the server again and my friends and I can join :)I did change nothing since thursday and now it is working just fine and I am not quiet sure why.
but in the end I can play again with my friends :) So thank you so much again for your help! Great support!