Resource temporarily unavailable: AH00480: apr_thread_create: unable to create worker thread

I am getting this type of error:

Resource temporarily unavailable: AH00480: apr_thread_create: unable to create worker thread

after my provider suggested a migration from CentOs to Ubuntu.

The backup/restore of my accounts was done, the files and overall DA structure are there, but Apache is unable to serve the files.

Here are a few more lines from the Apache Error Log:

[Wed Jun 09 05:14:03.288163 2021] [lbmethod_heartbeat:notice] [pid 744:tid 139680405668288] AH02282: No slotmem from mod_heartmonitor
[Wed Jun 09 05:14:03.681808 2021] [mpm_event:alert] [pid 783:tid 139680405668288] (11)Resource temporarily unavailable: AH00480: apr_thread_create: unable to create worker thread
[Wed Jun 09 05:14:03.682845 2021] [mpm_event:alert] [pid 789:tid 139680405668288] (11)Resource temporarily unavailable: AH00480: apr_thread_create: unable to create worker thread
[Wed Jun 09 05:14:03.683254 2021] [mpm_event:error] [pid 744:tid 139680405668288] (11)Resource temporarily unavailable: AH00481: fork: Unable to fork new process

There is more, but before flooding the forum I have a simple question (* a silly one perhaps, but I am only an end user = can move around a server and do some minor changes, but I lack real knowledge of Unix systems):

---- does it make any sense to try migrating a DA installation from CentOS to Ubuntu in the first place?

My provider's idea. To me, it doesn't look like it can function = see above.

There are consequences on my email accounts too. I get errors that are apparently password-related, but I think memory is the real issue because I had the same problem occasionally when some other issue caused memory drains in the system (* solved by a reboot).

My original VPS runs on an old CentOS 6, so I understand my provider's goodwill to orient me to an OS that will not be in an end-of-life situation again in a couple of years.

But I'm afraid that trying to fix a broken Apache/DA interaction will just make me waste time.

Any suggestions?

Thanks in advance.