Does your host have some form of security measure in place, securi firewall or similar? Not sure why it would do that unless something security/permission/error(possibly reporting) was coming into play.
As a work-around I suppose you could install this mod:
https://vborg.vbsupport.ru/showthread.php?t=257072
Export the custom templates into a .php file, then edit that file directly (using Notepad++) you will see where the template is within the actual php code

. Once you're finished (save then) import the template php file to overwrite the current style with your new changes

.