After updating our server to Minecraft 1.20.5, we encountered an issue where interaction with certain villagers was not possible (cf. attached video). When we first loaded the world, the logs displayed the error message "Failed to load offers: Item must not be minecraft:air" (cf. attached screenshot #1) and attempting to interact with the affected villagers resulted in the same message in the logs.

After reloading the server, the affected villagers lost their trades but not their levels (cf. attached screenshot #2 ; the librarian lost its trades while still being a master).