Fatal error after plugin activation at wp-includes/functions.php

Hi, I have got the three following errors:

Warning: fopen(/home/halftheb/staging/27/wp-content/plugins/user-blocker/user_blocker.php): failed to open stream: No such file or directory in /home/halftheb/staging/27/wp-includes/functions.php on line 5782

Warning: fread() expects parameter 1 to be resource, bool given in /home/halftheb/staging/27/wp-includes/functions.php on line 5785

Warning: fclose() expects parameter 1 to be resource, bool given in /home/halftheb/staging/27/wp-includes/functions.php on line 5788

My plugins are all up to date. Please check, if this bug is related to my installation or a general issue. If it is just appearing at my server, I will do further research.

Best regards and thanks in advance