I am struggling to get my setup working on my iPad. Especially the mail/contacts/calendar option. Tried to use the Apple profiles from within the web interface of mailcow. Which seems to be damaged. At least I get that error output. When trying to set a calendar manually I also get an error.
Have you tried adding them separately? Settings->Calendar->Accounts->Add Account->Other->Add CalDAV Account->Add mailcow server domain, your email address, email password
Can you upload the mobile config file?