This problem occurs after 1-3 days of apache start. After restarting apache, the error / warning does not occur again, but after some time or several days, the error occurs again. Until now I did not know what the cause of the error are.
Here is the error log, I got ssl:warn on my error log:
Invalid argument: AH02026: Failed to acquire SSL session cache lock
I use Apache/2.4.29 (Ubuntu 18.04). I use http2 and ssl-stapling.