Cannot connect to another user on the same hub.

Recently, I tried connecting to another user on my hub, and the connection "succeeds" in that his profile is added to my connection list but with the little red emblem next to it. He never receives any connection request. When he tries to do the same happens but the other way around; I never receive a request.

We're running Hubzilla 4.0.

Error log shows this:

<redacted> AH01071: Got error 'PHP message: PHP Warning: json_decode() expects parameter 1 to be string, array given in /var/www/hubzilla/include/follow.php on line 126\n', referer: https://hub.rootfs.eu/connections

Not 100% sure if this is related, but looking at the file name, it might very well be.