[Problem] - Text boxes in control panel disappear

[MKZ Industries]

Hi all, 

First of all sorry if this doesn't go here .

I have just installed back PS 1.7.7.8 due to incompatibilities of my theme with the new PS 1.7.8 (for example, my search has messed up) only to found that all the text boxes of the control panel disappear! (see attached pictures: one for the T&C configuration page, the maintenance menu and the third one from category description)

They appear for one second, you can barely read the text and then they disappear. I have tried reinstalling it, also removing and adding the languages... but cannot solve it. 

Any idea would be much appreciated. 

Best regards,
Xabier