I upgraded the firmware on my Vigor2820Vn from 3.3.5.2 to v3.3.7

After upgrading the NAT > Open ports page was blank except the << 1-10 | 11-20 >> (and the ports that were meant to be open weren't).

I downgraded back to 3.3.5.2 and the open port configuration had survived except the WAN interface parameter had changed from from WAN2 to WAN1.

I don't know whether it was just me and I should try again or whether there is a fundamental problem with the upgrade?

The 'fundamental problem' with any re-gradesbetween firmware versions, is, that, for best results you should use the .rst ( meaning Reset of all config ) file and whizz through and put in your config from scratch. Backing up and restoring settings isn't particularly portable between different versions of firmware. Sorry, it probably isn't the news you'd like to hear, but it should save a lot of gripes.

Best regards, Neal