Weird, weirder, weirdest ... I just tried something else.
- Just keep the user in OS.
- In Profile Manager I removed the Mail payload all together and saved it. Well actually I hit the "-" sign in the Profile Manager next to the 'Settings for <user>''. But I only use one payload, so its fine for me. Then you cannot download the profile anymore, but that's ok.
- Immediately create the Mail payload again for that user in Profile Manager. Save it (it is the first Save again, so the button says "Save" instead of "Save...")
- then download it .. and the profile loads again successfully in System Preferences.
If you repeat this, then it only works if you first delete the profile in System Preferences for that user. Otherwise I get the same error.
So to sum it up it looks like it basically comes down to this:
- the Mail profile needs to be created a first time (it will say "Save" instead of "Save...")
- and the profile must not already exist in System Preferences.
I really don't know if this a strange workaround and keeps everything working. But I have to find that out. I don't think (hopefully) it will remove existing mails for that user, because I'm only changing the payload in the profile. And removing/adding the profile in System Preferences.