Ideas
I also have the same problem, I have mods but I removed them, repaired the game and then fired up a saved game that I use for this purpose (testing the game without mod), and I even created a new game completely and same problem happens. If my Sims go on date they can't get seated but if they go alone or in twos or even as a group there is no problem. This is both with ask on a date and asking a Sim to create a date.
But there is a modfix in a bigger mod that NeutrinaEditora (thanks so much!) mentions earlier in this thread, that is working very well for me. It's Carl's Dine Out Reloaded mod and my Sims can have a date at a restaurant again. I have used it before in the past because the Dine Out pack started to flake out on me and the waiters refused to bring the food after taking the orders and it fixed that problem. I had just completely forgotten about it after I got my new computer and revamped my mods folder.