Error after V7.0 upgrade - by: P.Lego

We get an error message when we try to create or copy an activity.

log file content :

2018-04-20 07:51:55.454 ===== TRACE ===== NEW CONNECTED USER 'Philippe'
2018-04-20 07:52:14.511 ===== TRACE ===== Cron started at 20/04/2018 07:52:14
2018-04-20 07:54:30.455 ***** ERROR ***** [V7.0.4] ERROR *****
2018-04-20 07:54:30.456 ***** ERROR ***** [V7.0.4] on file 'C:\inetpub\wwwroot\projeqtor\model\Mutex.php' at line (54)
2018-04-20 07:54:30.456 ***** ERROR ***** [V7.0.4] cause = count(): Parameter must be an array or an object that implements Countable

Thanks for your help.