Do you have more information on how you imported the files and where they are located in your system?
Do you use enocre to export the files?
Sounds like one of your clips is corrupted. You might try copying your project over to a new file and using the copy, delete the files in your project one at a time until you figure out which one it is. You may need to repair or replace the offending clip.
Afterwards I added titles and maybe someting else and now it is not exporting anymore
Since the Project Exported fine before, and you have Deleted the Titles, then a likely condidate would be "and maybe something else."
Steven's suggestion might be able to highlight that "something else," and that could well be the cause.