Hi,
If the system is going down in 15-20 min then what message do you see in the dev_w* logs.
Have you checked sm21 logs ....
Have you checked db alert.log.
How much memory is configured for the DB.
Are the archive logs backup happening properly.
For the license error can check the below note as you mentioned you only got it once.
1717857 - Protection of licence key memory
Thanks
Rishi Abrol