I added portal crown blocks to each side of the portal keyholder block, but when inserting an advanced key, it says "You must add two Portal Crown Blocks to the top of the frame". Am I doing something wrong?

I'm sorry for the late reply. I was on vacation.

Are you using a cosmetic mod that affects walls and fences? I have seen seen a mod achieve diagonal walls by replacing all walls with its own version. Then the blocks next to the keyhole are no longer dimdungeons:portal_crown_block, but something like betterwalls:betterwall (or whatever).

I forget which mod it was but the solution was to add Dim Dungeons to the blacklist in their config. So that the block wouldn't be replaced.

If the blocks next to the keyhole are still from Dimensional Dungeons then please focus them with a F3 screenshot and I can debug further.

Thank you for playing my mod!

That was exactly it! I'm using Diagonal Walls and just noticed that the crown block is changed to diagonalwalls:dimdungeons/block_portal_crown

Thanks a lot for taking the time to check this out, and lovely mod btw!