Slime stopped spawning in slime chunks.
I cleared out four adjacent slime-chunks (levels 11 - 13) and until recently there always spawned slime. Now it doesn't anymore, at all.
Restarting the game or my computer does not solve this issue.
Other spawns seem to be working (my mob farm still works).
OS and game are up to date (I checked this yesterday)
I play 100% vanilla survival on normal difficulty with no cheats enabled.
Seed: 14159265
I found the source of the issue: there were too many cows above the clime chunks. After "removing" some of the herd, slime did spawn below again.
Case closed.