PROBLEM Cannot activate plugin

[iaeon]

Website health check?

Also check data/other/logs/errorlog.txt  or enable GSDEBUG in gsconfig...

Is it a fresh install? Have you moved the site?

After some puzzling I found that PHP needed to be run as an Apache Fast CGI application for things to work OK. 

The site had been migrated some time ago and this problem was encountered when trying to make some updates.