Just in case this helps anyone else.
I've finally been able to upgrade from v4.4.3.1 to v4.4.5.5 keeping my settings without the router rebooting when I log in!
After doing a backup of them I cleared all the various settings that had their own backup option, like VPN, Objects etc.
Rebooted then upgraded to v4.4.5.5 which worked and didn't keep rebooting as soon as I logged in.
Restored each sections settings one at a time with a reboot after each restore.
I found that the Lan to Lan settings were the ones that caused the router to keep rebooting after logging in, even though none of them were enabled.
I had a number of Lan to Lan entries, all of which had been working for years under v4.4.3.1 and earlier firmwares and hadn't caused any problems with any previous firmware upgrades.
I reset the router to factory, flashed it back to v4.4.3.1 and restored the previous configuration.
I deleted 5 of the Lan to Lan settings which aren't used very often, upgraded the firmware to v4.4.5.5 and the router now works without rebooting when I log in!
The deleted Lan to Lan entries were no different in configuration to the remaining ones, except for user names and passwords so I've no idea why they were causing a problem.

So far the only thing I've found that is different is the Lan Fixed IP Comment is now limited to 11 letters instead of 12!

Andy wrote:

You can download it using the link from the main downloads page...
https://www.draytek.co.uk/support/downloads/vigor-2927/send/896-vigor-2927/3430-v2927-4455
 

But when I try to install that, the router reboots to Firmware 4.4.5.2, as if the 4.4.5.5 install is faulty, and is being automatically reverted to the earlier firmware. Any thoughts?

That does happen sometimes, unfortunately.

I just try from another node and/or browser until one is accepted as stable enough to make the update succeed.


I have even had failed attempts from a local network node and succeeded with a ‘Remote Dial In VPN’ connection instead; it isn’t uncommon to encounter what you’ve experienced.

@HodgesanDY Thanks for your helpful reply. As it happens, the upgrade was showing in the router web UI this morning, and installed normally from there.
Thanks again. DA

Cheers for your observations and perserverence with this, I've just tried going up from v4.4.3.1 to v4.4.5.5 again and the same reboot loops persist.

I have about 30 LAN to LAN entries, all of which are in use on one router and only two in use on another Router (so you can guess which one I'll upgrade first). So in essence it's the LAN to LAN VPN entries, or some of, that are causing this issue? Did your GUI show correct information, ie active ports, virtual LEDs active, on the top after upgrading without the LAN to LAN VPN's restored?

On the LAN fixed IP side of things, has restoring, after clearing before upgrade, avoided the text corruption (with just the last original character missing) or are the entries still corrupted?

After upgrading without any of the Lan to Lan VPN profiles everything appeared to work as normal, LED's, Dashboard, Status pages etc. After restoring the Lan to Lan profiles, less the 5 I deleted, everything kept working.
The Lan fixed IP address Comment was still corrupted after restoring, even with all of the various sections that have individual backup options being cleared first. Once I edited the Comments to correct them they haven't been corrupted again.
The router has been stable since I posted what worked for me.
The only things I have noticed since are the Quality Of Service page gives an "Authorization Error !!!" error and the Auto Logout randomly logs you out even if you set it to "Off", which never happened on previous firmwares. There may be other problems on settings pages that I haven't needed to use yet. I have looked at all of the pages, but not made any changes to them, and I didn't spot anything obvious. The Dynamic DNS entry is working and updating correctly.
I haven't yet had the need, or the time to, to recreate the 5 Lan to Lan VPN profiles I deleted to see if one or more of them causes a problem again. But apart from the passwords, profile name, preshared key and the Remote Network IP addresses they were all identical to the remaining profiles that didn't cause it to reboot. All of them, including the deleted ones, have been there, fully working, for years and have not caused a problem with multiple previous firmware upgrades.