I don't have some niche weird calendar. It's just a NextCloud account that syncs on literally everything else I have. It synced with Android with 0 issue, it syncs on all my Linux laptops - Using Kontact or Evolution or the PIM add on.... Ah, there missing on this laptop cause I forgot to enable it. Boop, there's my appointments, easy peasy.
On my FLX1... CONSTANT sync failures. Or like what it's doing now where I can't even save an appointment. It acts like it does, and just eats it.