Minecraft Vanilla Server Microsoft?

I have a question…

Today I wanted to play Minecraft with my sister on our vanilla server via Hamachi. Everything as always.

My sister, however, only had Minecraft on the Switch before and we bought her for her PC. However, when logging in, she always gets the message Error: Null

I've already tried everything; it's not the firewall or the internet either. If I try your account on my PC, it doesn't work either, but mine does.

My friend also plays with a Microsoft account and there are no problems.

Can someone tell me why this could be?

Here is a picture of the message my server brings.

Try it in the server.properties online mode = false
Are you using the Bedrock or Java edition?
The two are not compatible with each other.

Does your sister have Bedrock or Java version?

Can you send the whole bug back please? You only see half -.

Thank you that worked with the Server.properties ^ - ^ does that have any other effects?
By the way, we use the Java version.