I am sure some would suggest to start with replacing your scripts.
Since you are on version 7.5.5 You do know there is a security risk on that version. And you should upgrade.
But anyway.
The simple would be if you have member access to files. Or at least access to your same version virgin zip file.
I would follow the upgrade route for the same version.
Meaning 7.5.5 replaced with 7.5.5 and follow the upgrade manual
for not replacing your styles directories etc.
Of course in this upgrade there is no need to run the upgrade script.
But it would at least assure you you are running stock code.
If you have never modded the scripts then this would not impact you what so ever.
And would rule out at least your scripts causing the issue.
