I've been stuck in 'Riders on the Storm' where I couldn't save after accepting the mission, and I've read posts from other players with the same issue on other missions. Well, I've found a workaround that might work for others too. The issue only happened if I fast traveled after accepting the mission. If I completed the mission without fast travel, there were no save issues, and after completing the mission, I could freely fast travel and save again.
Hope it helps.
Hope it helps.