Joining a custom server requires to be logged in with a Microsoft account.
In this case it is a child account under parental control.
Since 1.21.120 the join process results in (freely translated from German) “Your client has issues connecting to the multiplayer services. Please check your internet connection. Multiplayer connection failed.”.
Windows 11 device (parent account) can connect to the server and works flawlessly.
This behaviour strongly reminds me of MCPE-230762 and MCPE-230429 which seem to be about some malfunctioning authentication flow introduced in 1.21.120.
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However, this issue is Invalid.
An account is required for online play.
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