Oh, well, since no one specialized offers any help, I will offer myself. This is - or should be embarrassing - to let a lawyer who knows nothing about coding to solve the technical problems caused to other ppl by your own hack.
After 30 minutes of testing the most weird things on the test forum, I did as follows:
1. I created the logfile_worms.txt and uploaded it in forum root.
2. CHMOD-ed it to 0666.
3. Disabled the addon and noticed that the error was gone.
4. Uninstalled the addon - the error still gone
5. Deleted the damned .txt file from the root and felt very proud of myself

)
Hope this will work for everyone.
Extremely disappointed of this type of lack of responsibility.