So you need to download the nimbus folder [that comes with the .3dsx version, but not the cia version] on their github repo and add it to sdmc/3ds/. After, re-open nimbus, it should say an update occurred and to press start to restart. After restarting, open nimbus, press y to...
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enable plugin, and the error should stop showing at the station when trying to connect. Hope this helps!
P.S. be sure to open your gate by asking the porter to invite visitors so people can visit; otherwise you will not show up on your friends available towns
Thanks for the info, helps me a lot! ;)
I used the cia zip folder and it worked for me. I think it depends on whether nimbus is installed in your home menu or the hombrew launcher (or both maybe?).