[TYPO3-mvc] sr_feuse_rregister and extbase Extension

[Astrid Haubold]

Hi list,
Iam using sr_feuser_register v2.6.1 and extbase v. 1.3.0 with TYPO3 4.5.1.

In my extbase extension 'arztcme' I am adding fields to fe_users. I want 
users to register with sr_feuser_registration. For that I want to 
display all the additional fields from my extension in the registration 
form.

I followed the documentation for sr_feuser_registration and added the 
following lines to the ext_localconf.php of my extension:
if (TYPO3_MODE=='FE'){
	$GLOBALS['TYPO3_CONF_VARS']['EXTCONF']['sr_feuser_register']['extendingTCA'][] = 'arztcme';
}

My fields are all displayed now but also I get the following error message:
Warning: array_key_exists() expects parameter 2 to be array, null given 
in ... /typo3/sysext/extbase/Classes/Utility/Extension.php on line 214

The error seems to have something to do with backend Module 
configuration?? I have 2 backend Modules registered in my extension. 
This was done by the extbase kickstarter.The error does only occur on 
second display after configuration cache have been deleted and 
afterwards: the first time everything is fine.

Does anybody have an idea what could be wrong?
Thank you very much in advance!
Astrid