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First I thought, there was a problem with some TYPO3 cache files, but after cleaning the typo3temp folder, the problem still occured. I then looked into to Webservers Error log and found the following logentry.
PHP Fatal error: Cannot unset string offsets in /path/to/typo3/sysext/em/classes/tools/class.tx_em_tools.php on line 382
After some research I finally found the solution to the problem, which is the result of a bug in the TYPO3 Extension Manager, which has been fixed for some days ago.
I manually opened the file "ext_emconf.php " of every installed TYPO3 extension and checked, that the constraints-array was correct. In my case it was powermail which had a wrong entry in "ext_emconf.php".
I had to change:
'conflicts' => '',to this:
'conflicts' => array(),
After I fixed the wrong entry, the extension manager worked fine again.