Well I'm a newbie don't expect from me an expert adviceBut while I was searching KB I found something that might be useful for you to start with
https://kb.mailwizz.com/articles/email-templates-broken-uploadupdate-can/
NoHow did you do the changes? I hope not in the files...?
Table name list_page here you can find templates updated for each list.But would need to know which table to check, pls.