can't enter dungeon more than once?

after completing a dungeon once, when I try to enter again, I get an error message. something about quest log full....anyone else got the same issue?

 

The notice indicates the reason. You have to give up some quests.

 

As per your picture, it looks like you have 20 or more ongoing quest. You have to give up a few to enter the dun.

 

But I had the same number of quests already accepted accepted when I entered the dungeon first time for the day, and there was no issue. It is only when I try to enter again, it gives this error???

 

My only guess the game algorithm did not flag it the first time round.
I did experience it myself what happened to you. It did not flag me entering dungeon when my quests exceeded the limits. Only upon me trying the 2nd time did I realised what was happening.