I'm running 20.7.4 on a ProtectCLI 6-port device (6 Intel 82583V Ethernet NIC ports). I'm using 3 of those ports. 2 are different WAN ports, and one is my internal LAN port. I have my home network running as the native VLAN on the LAN port, but also have a couple VLANs going to my switches that I use for various purposes.
LAN port is "em1".
I yesterday, I needed to create 3 VLANs for a lab environment to set up some equipment. I went to "Interfaces", "Other Types", "VLANs". Every time I added a vlan to em1 and clicked "save", all traffic in/out of the LAN port stopped and I lost access to the router. I had to physically shut down the device via the console, and upon boot, the new VLAN was available and I was able to complete setting up the IP addresses, DHCP, etc for that VLAN. This happened every time I added a VLAN.
So - I upgraded to 20.7.5, added another VLAN and this time I did not have to reboot and everything just worked. Not sure if there was a bug in the older version or what. Eh. :-\
Never happened to me, logs would be interesting. Maybe you run IPS or Sensei in LAN?