In case somebody else hits the same problem:
Starting with kernel 5.12, I wasn’t able to install NVIDIA drivers anymore. Installing failed with lots of undefined symbols, e.g. "ERROR: modpost: “pm_vt_switch_unregister” undefined!
I did pretty intensive research and found out that setting kernel option CONFIG_TRIM_UNUSED_KSYMS caused the trouble. After disabling, driver compilation worked fine again.