Gearbox lockup / seize up when shifting out of 1st gear

Strange thing happened twice today and I have no idea what could cause it.

While driving, when I shift out of first gear on my way to second, the gearbox locks up so that the rear wheels start skidding and I come to a stop.

While skidding, the gear lever is stiff.

Once stopped, it relaxes after a short while and I can select gears again and it works normally.

It is as if the gearbox selects another gear before it is out of first. How is this even possible?

Its a 2001 1.8 manual with 406,000 km.

Taking a chance and asking here, because I am trying to figure this out fast, skidding to a stop in traffic is not something I enjoy doing too regularly.

I looked through the 22D section of the manual for the gearbox, but no ideas spring forward yet…