This bug is as old as the game. Just go to imperial city and then queue again to get back to cyrodiil.

Almakor wrote: »

I've played since beta and this has never happened to me.

It's a known bug. It's because when you actually go INTO those delves, you're being taken out of the campaign. It's happened to quite a few people and it's been reported quite a bit. Here's just a few of the threads;

(2014) https://forums.elderscrollsonline.com/en/discussion/107091/cant-leave-cyrodiil-dungeon-the-destination-is-full
(2015) https://forums.elderscrollsonline.com/en/discussion/177654/destination-is-full
(2016) https://forums.elderscrollsonline.com/en/discussion/361679/cant-exit-cyrodiil-delve-destination-is-full
(2020) https://forums.elderscrollsonline.com/en/discussion/510281/problem-with-using-delves-in-cyrodiil
(2020) https://forums.elderscrollsonline.com/en/discussion/507795/stuck-in-a-delve-in-cyrodiil
(2020) https://forums.elderscrollsonline.com/en/discussion/525824/cyrodiil-delves-getting-stuck

Not to diminish your frustration, but being here since beta - the bug traces back since 2014, which may as well be beta at that point because not much has changed with Cyrodiil delve instancing between Live and Beta.