Alright, hopefully this will be enough information to help.
When completing any quests, required or side quests, I have fewer or no quest rewards. When my boyfriend was hosting our online game, I stopped being able to complete side quests because I couldn't select from the available rewards, because there were none. I had to open the task manager and kill the game to reload, as there was no way to close it. Upon re-entering, the quest was no longer available to complete, but I also had no new items in my inventory - I'm not sure if I had any more exp or fame, as I thought it was a one time issue and didn't note those numbers. This first happened in the mercenary camp in the Frozen Steppes, their little hideout sub-area. The people being rescued were the first quests I had issues with. After that, I started hosting, and was at least able to complete quests, but would only show 1 quest reward or sometimes none, but my boyfriend would have the option to pick from 3. All of the quests are from the Estherian Enclave areas, as these are our first characters with mods.
I originally had the nether imp pet, but have since transformed him into the improved dragon, though the issues started before the transformation and have continued since.
I hope this covers all of the questions needed, and I'd really appreciate any ideas about what could be happening. Thanks!