TYPO3 Error "Sorry, but an error occurred while connecting to the server." in Extension Manager

After the latest security updates for some TYPO3 extensions I was about to update those extension in a TYPO3 installation. After opening the extension manager, TYPO3 (version 4.7) did show the following error message:

Connection Problem
Sorry, but an error occurred while connecting to the server. Please check your network connection.

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.