I ran into this issue after spending literal hours editing a lot I downloaded from the Gallery. At first I thought it was the Gallery lot until I re-downloaded the original unedited lot and had no issue loading the restaurant. Therefore, I knew that it HAD to be a CC item that I used to decorate my lot with that was breaking the game.
I saved my edited lot and used the Tray Importer to see what CC I used on the lot. I then went back and deleted the CC items from the lot room by room and reloaded the lot until I was able to narrow down the room that had the broken CC. After much trial and error, I FINALLY found the problematic CC that was breaking the restaurant.
It ended up being the Cornucopia buffet table created by Coolspear (Cool1) - specifically the "Starry Eyed Collection Banquet Table - Featuring the SGL Cornucopia Enhancement" that he cloned from the High School Years EP. I tried to load the lot with another one of his buffet tables that was cloned from the Luxury Party stuff pack, and the lot loaded perfectly fine. So, it is just this particular modded HSY buffet table that is broken in restaurants. I didn't bother to test if this table works with other venue types, I simply opted to delete it from my mods folder entirely.
So, if you play a modded game and are running into the 123:20ece5dd:23121b4b Error Code, it's most certainly a piece of borked CC on the lot.
Here's a link to the Sims 4 Tray Importer https://luniversims.fr/files/file/73-sims-4-tray-importer/ . This is your best tool for figuring out what's breaking a lot. You will need to save the lot to your Library first in order for S4TI to list the CC.