Using Classic Outlook on Windows for personal email, calendar, and contact management
Not that I've solved it for myself completely, but might be a hint for someone.
In my case, there seems to be a problem with one specific action: mark as read. even though named the same, but there's compatibility problem between classic Outlook and new / web outlook interface. I can avoid this action, so no more problems for me.
I have only few simple rules (find by sender/subject/recepient known useless emails and mark as read / delete).
In new outlook 2 out of 3 rules stopped working. And message displayed for particular rules: "This rule can't be edited or viewed in Outlook for Windows". Similar behavior in web Outlook.
Switched back to Classic Outlook. Found out, that 2 rules (that previously stopped working in new outlook), had action 'mark as read'. And when this action used, and rule is saved, classic Outlook warns that rule works when client is running (probably action specific to Windows client app). OK, I get it. Removed the action, saved rules, switched back to new outlook.
All works! Now in new Outlook interface, for one of the rules tried add step 'mark as read' - no problems, added. Checked in web outlook - no problems with this rule. Good!
Switched back to old Outlook - got error: "There was an error reading the rules from the server. The format of the server was not recognized." None of rules are shown (even if only one probably has conflicting action)