I have upgraded an EE 1.6.8 website all the way to the latest EE 5, but now I have a problem with saving templates. When I try to save a template after editing, the homepage shows up and the template is not saved... it's not a redirect to the homepage, because the url still points to system/index.php?/cp/design/template/edit/16&S=c81740cc76a21628dcf01628398632f0
The weird thing is this doesn't happen to any template, some work fine, and there doesn't seem to be any logic in which work and which don't.
Any help is appreciated!
EDIT: When the template contains one of the following strings it refuses to save:
<!DOCTYPE
<html
text/javascript
<meta
<link href=
<?