Subsite admin does not have promission to view wp-admin

I have multiple subsite-admins complaining that they can not acces their websites. They get the notification:
Sorry, you are not allowed to access this page.

One case is with a new and fresh website that is requested with the pro sites plugin. Other one is a website that is up for a while. However he got no access since we mapped this domain with the wpmudev domainmapper. All other admins have acces.

I have no single clue here, what is going on..