PROBLEM Can't modify, update pages on new installation (all 755)

[shawn_a]

host error log ?

gs health check throw anything suspicious ?

What if you do something like toggle a plugin active inactive?

Check or remove your .htaccess make sure the rewrite base is correct for a subfolder, it is a bit hard to debug stuff in subfolders also. If not comment out the rewrites in the htaccess and try