[TYPO3-seminars] MySQL error when clicking the register button

[René Bing]

Hi all,

there is some strange behavior of my seminars installation, when i click the 
register button and the page with the onetimeaccount extension is loaded:
---
Warning: mysql_fetch_assoc(): supplied argument is not a valid MySQL result 
resource in /typo3_src-4.2.1/t3lib/class.t3lib_db.php on line 808

Warning: mysql_free_result(): supplied argument is not a valid MySQL result 
resource in /typo3_src-4.2.1/t3lib/class.t3lib_db.php on line 835
---
Typo3 Version 4.2.1
Seminars 0.7.1
onetimeaccount 0.3.0
ameos_formidable 1.1.2

The database analyser doesn't report any changes. But there are several 
broken database relations i cannot understand. I didn't delete any records 
in the database. If i open a seminars record in the backend there are no 
errors at all.

Do you have an idea/solution for me?

Regards
René