I updated only NC, no PHP update or anything and the error occured.
I went back to NC 21.0.2 with a backup and now it workls fine again.
I did some tests. I did a fresh installation of 21.0.3 on a second DiskStation (another Firmware version) and as soon as I enabled APCu the cron stopped. I did then a dffresh installation of 21.0.2 and the croin stayed intact after enabling APCu.
Here is the solution for Synology:
Create the file /usr/local/etc/php74/cli/conf.d/user_settings.ini with following content:
extension = apcu.so
[apc]
apc.shm_size = 512M
apc.enable_cli = 1
Thx to the guys from the German Synology Forum.