Minecraft v1.15.2
Forge v31.2.31
Magic Mirror v0.2.2.350

After right-clicking the Magic Mirror with a Pillager Banner, my server immediately crashed.
Crash Report

Ouch. Sorry about that.
I will investigate tonight.
I suspect there is some code that is running on the server that's intended to just run on the clients.

Thanks!

Sure! I'll see if I can give it a try tomorrow.

Unfortunately I've had some trouble reproducing this issue, but I'm pretty sure I found the cause.
Going to release a fix now. @WenXin20, I would really appreciate it if you could verify whether this resolves the issue for you.

Let me keep this open pending feedback.

Since CurseForge approval of new versions sometimes takes a while, you can also find the latest version for 1.15.2 here

I was able to apply the banner without crashing, but when I broke the mirror with the banner applied, the server crashed.
Crash Report

Ugh. Okay I wonder why it's working for me... Might have to do with the way I have my testing environment set up.
Pretty sure I know the cause for this one as well. I'll try to get another fix out tonight.
Thanks for testing!

So the method I'm using is BannerBlock.forColor, to spawn a banner of the right base color back into the world.
For some reason, this is marked for client-side use only. No idea why. Especially considering it's the only accessor of a map that is filled in both on clients and servers.

The workaround for this one is going to be quite annoying.

I haven't got anymore crashes, so it appears to be fixed. :)

I think I fixed the crashes for real now. If it's still happening, please let me know.