Strange behaviour occurring with ports...

[Ukendt]

Sabnzb v. 4.3.3, not win service, windows 11, 4 months old pc
CPU: Intel(R) Core(TM) i7-14700KF 3.40 GHz
Nvidia 4080S 8GB
Ram: 32Gb
HD: SSD 1 TB and 6 TB HD
No VPN, cabled internet

Suddenly, between 2 downloads I get:

Fatal error:
Uanable to bind to port 8081 on 127.0.0.1
Some other software uses the port or Sabnzb is already running


Since this episode, Sab will, still automatically, get new nzb files from monitored folder
but, if I click on a nzb file in any other folders, I only see this error.
Moving files to the monitored folder makes Sabnzb react with no excitation and correctly DL all the monitored files.
If I close the service right clicking on the yellow icon (process line) and then I click on a nzb file in any folder, Sabnzb will start working as it should, but if I click on a new file in any folder, while Sab is working I'll get the very same error. I have to close the service to make Sab start over again
and work with any nzb files from any other folder (one file at the time, shut down service, and so on and so on)
While Sab is running and the yellow icon appears in process line, the program will work fine detecting and downloading files from the monitored folder.

This behaviour is new to me.
Before this, Sab was always running in the background and I could BOTH download automatically files ending in the monitored folder and (at the same time) click on any other nzb files in other folders without probs (new files been placed in the queue)
Of course, it doesn't help changing the port to 8082 or even 8083.
Sabnzb is not blocked by firewall or AV (I only use windows, original programs and nothing else)
Any clue?
Regards