Are you using the latest version? That’s a bug which was fixed and patched a while ago. 

Last patch that appeared today was not installed. Every single other update was implemented. I just updated to latest now, and tried again, seems it worked or did something different.

So "a while ago" definitely not, only if the patch was the december I just installed after this error appeared.

It was patched 28 days ago. I guess you upgraded to 4.7.4 before the patch was applied and you haven’t visited your ACP Support page since then which would have mentioned that a patch is available 

Explanation that sounds reasonable, thanks for the reassurance that all is good now!