Please provide your PMS logs after restarting and waiting 3 minutes without doing anything.
I had the exactly same issue on my Raspberry Pi 4 hosted PMS for several days. The issue disappeared when I set Limit Remote Stream Bitrate (from original Unlimit) to 20 Mbps (1080p)on the Settings - Remote Access dialog. I am on Verizon FiOS and running PMS1 on RPi4, PMS2 on RPi3B and PMS3 on Windows 10.
My PMS on Raspberry Pi 4 worked for a few hours after setting Remote Stream Bitrate, but its remote access stopped after a few hours. Unset Remote Stream Bitrate to the default (original, unlimit) and enabled remote access, this time it worked. Conclusion: please disregard my post. It seemed that setting Remote Stream Bitrate has no bearing to the topic issue.
