So, can't seem to find anything very concrete either. but here's Bing's suggestion :D
The 504/1046 error code in Microsoft Teams occurs when a user sends a 1:1 chat message to an external user. The error message indicates that the message could not be delivered to the recipient 1. Unfortunately, the exact meaning of this error code is not documented by Microsoft, and there is no official solution available. However, some users have reported that disabling external communications in the Teams admin center for a few minutes and then enabling it again can solve the issue 2.
If you need further assistance, I recommend visiting the Microsoft Tech Community forum or contacting Microsoft support directly.