Did you do anything with the files or renamed anything?
Please post your URL, Username and Password in Secure Tab and i will take a look and maybe figure out what the problem is.
FTP info with Username and password is also necessary ...
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The problem is in custom html particle in header when you take out " " from the class the save will work i tried this on my own install and its working fine so its isolated case!
I upgraded the template and gantry to 5.2.5 and that didn't solve anything i have one thing to ask you to do backup the custom directory and erase it then go to the layout and save it and then add custom html to header and that same code in it and see if the error will happen again....
Wow that is weird. You are right about the quotation. I will try to do as you suggest sometime tomorrow but that is the issue. what's really odd is that the custom html is still rendering my class without the quotations. It's not cache either as I cleared both browser and site.