TYPO3 Core

"Sometimes" is a very vague description.

What does sometimes mean? Are there certain conditions which are always met, when it does not work?
An idea: When you get this error, do nothing but change to an older version of the core and try uninstalling again there. Does it work then?
Maybe the string which holds the installed extensions in localconf.php is corrupted?

Do you get an error message (when you activate the display of error messages)?

Nothing happens when trying to uninstall an extension which requires Clear cache - it is simply not uninstalled.

Claus, is this the OLD or the NEW extension manager? Can you try again with beta4?

After upgrading to 4.5beta4 the OLD is gone - tried to uninstall an extension which requires "Clear cache" and it crashes :-(

See attachment (screen.png)

Thanks for information, now I was able to identify the duplicate:
http://forge.typo3.org/issues/12218