Fatal error on activation

Hi there,
We’re using WF premium on several websites and everything worked fine by now. However, today we have noticed that it’s not activated on one of the multisites we have.
When I tried to activate it I got “cannot be installed due to fatal error” message. There is an empty iframe under the message where one would expect to see some error description.
I’ve tried to uninstall all other plugins and then activate it again. Didn’t work.
I’ve tried to remove the plugin and delete all wp_wf… database tables and then to install it again. Didn’t work.
I’ve increased memory_limit to 1028M. Didn’t work.
I’ve changed all permissions to 777. Nothing.
Do you have any idea on what more can I try in order to debug this strange issue.
Thanks

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