[TYPO3-calendar] Weird problem with extending cal

[Chris Müller]

Hello,

I am using TYPO3 4.7.5 and cal 1.5.1 and experienced a very strange 
behaviour: If I install the cal__dam_references extension from TER and 
logout of the TYPO3 backend I cannot login again. If I deactivate the 
extension I can log in to the backend again.

If I disable the cal__dam_references extension, the backend login works 
again. Okay, next step: I stepped through the ext_tables.php of the 
cal__dam_references extension and when commenting out the lines after

     // load TCA of db-tables

the login works again.

Okay, another try: commenting in again the lines and change the 
loginSecurityLevel in localconf.php. After adding a new user via the 
install tool to get the md5 hashed password I can login with enabled 
cal__dam_references extension.

Now I tried to reproduce this problem with a freshly installed 
introduction package - the same problem occured.

In the next step I created via the kickstarter a new extension which 
only extends the tx_cal_category table with a new field. The same 
problem occures: no backend login possible. After commenting out the 
loading of the TCA the login works again.

This is weird and I encountered this problem only with cal. I have no 
idea how to avoid this problem. And I don't want to use the normal 
loginSecurityLevel with md5 hashes.

Has someone an idea what causes this behaviour?

Regards,
Chris.