ORA-00845 after deleting files accidentally at /dev/shm
As described in https://www.krenger.ch/blog/ora-00845-memory_target-not-supported-on-this-system/ and suggestion of MarcoBaldelli /dev/shm just needs to have enough space. So the solution was to increase /dev/shm:

mount -o remount,size=4G /dev/shm


When MEMORY_TARGET is set to 0 (as in my case) it seems to be as following (see Oracle Documentation):

For the MEMORY_MAX_TARGET initialization parameter, decide on a maximum amount of memory that you would want to allocate to the database for the foreseeable future. That is, determine the maximum value for the sum of the SGA and instance PGA sizes. This number can be larger than or the same as the MEMORY_TARGET value that you chose in the previous step.