how to replicate the situation one of my characters is now in until a help ticket maybe results in someone moving my character:
1. enter the under-50 cyrodil campaign.
2. go in a delve.
3. hit 50 in the delve.
4. find yourself unable to exit the delve because it means trying to enter a campaign that doesn't allow level 50 in it
One other stuck situation in Cyrodil I found a while ago was:
1. enter a very busy Cyrodil campaign.
2. go to Sheogorath's little pocket cheese dimension.
3. find yourself unable to leave it because the Cyrodil campaign "outside" is entirely full so it won't let you in.