BubbleTea2020

The steps here did the trick: MC-227814 

The steps here will do the trick: MC-227814

The steps here worked: MC-227814 

Thank you, Tim! Your comment helped and more importantly - it worked. 

We have two PCs in the house, playing on LAN together. We had to do this on both of them and then it worked.

Same here - LAN doesn't work on Latest Release 1.17.

It doesn't work on previously created worlds (before the update) and it doesn't work on a newly created world either (after the update).

I get the same error for both - like the one in the screenshot. 

LAN worked fine on Latest Snapshot 1.17 and it worked fine on Latest Release 1.16.5.

Same here - LAN doesn't work on Latest Release 1.17.

It doesn't work on previously created worlds (before the update) and it doesn't work on a newly created world either (after the update).

I get the same error for both - like the one in the screenshot. 

LAN worked fine on Latest Snapshot 1.17 and it worked fine on Latest Release 1.16.5.