When you do a dungeon, don't party. No party will be more secure from any technical problem.
If you have partied with others, NEVER, I repeat, NEVER EVER leave your party. I had this problem before, but I learned it by myself. I tried with my friends in Forest and it works!

So, never leave your party when you partied with others in a dungeon!
And the guild list that sayin your friend "In Party", it doesn't matter because you will never can join in that dungeon.

Have fun.