Thanks Webnauts, but the problem is with verification within GWT. GWT says there is a technical problem at their end, but I have now verified two other sites since then and they did not cause a problem, so there doesn't seem to be one, otherwise the verification of the other sites would have been unsuccessful.

Maybe the problem is within the code of my .htaccess file? Not sure if it is good idea to post it here, but I could send you a PM.