Hate to necro an old thread, but I'm having the same issue verbatim.
After getting the letter, I stopped the storyline missions and spent a year running around doing missions for everyone in Denmark to make them all 30+ to help with the assembly. Then after getting most of the lords up there, I went back to the King and told him the letter was destroyed, and he told me to wait for the assembly.
I've tried walking up and down Denmark for 72 hours after talking to the king. I've tried running errands all over the continent for weeks before returning to Denmark to skip time for a week (ctrl + space). Tried to restart the game just to be sure it wasn't a client issue. I only had one mod at the time (VC Political Map OSP, just lets you see data maps of the status of various towns and such), which I deactivated prior to talking to the King when he says, 'wait for the assembly', and that didn't help either.
Don't know if it's relevant, but I still have the same mission log I had prior to talking to him. "Saving Your Neck: Wooing Farmers and Jarls" which only mentions the King having good relations with me (is it supposed to update as you make other Denmark Jarls like you?). None of the notes or quest logs have any notes that mention that I'm waiting for The Assembly to be called.
Are there any work-arounds or ways to force-trigger events via the in-game console or save editing? Because it would really suck if I had to abandon this entire playthrough just for one show-stopping bug.