The EU version has just had another BETA firmware release - 423RC4
Anyone know what's fixed & if a UK version is available?

It may be like the UK version that now has debug mode built into if (ask support) if it is the same its not got much in the way of fixes I believe but it does enable debug via the CLI so the logs can be sent to Draytec support via email to help them diagnose these ongoing issues. Sadly my wheelchair status has left me not being able to ad it as I had an accident and tipped out truing to access the modem, also it involves more DLM shenanigans possibly as you have to turn debug on run it and then off which means rebooting the unit a few times, that's what was mentioned in the PDF but in the long term would it be worth getting them the logs. Or hopefully it could be a fix, which I doubt just yet. I'm so close to going back to my old ECI modem but want to stick with the 166 as it should in theory be a good modem once sorted. Its just how long do you wait?

It would be nice if someone at DrayTek UK could setup a dedicated page for BETA firmware, perhaps as a sub-section to the existing UK Firmware download page.
At the moment they post BETA's for download but keep the actual download link secret, only giving it out via a request to the support dept.
Also a changelog would be useful to identify what's been altered, other manufacturers have been doing this for years.

Most companies don't post betas publicly in case it causes more issues, unlike beta hotfixes for security etc, I am guessing thats why they are doing this. I have to say I bought this Modem in January and its been a total disaster. I am wondering if its the intel (lantiq) chipsets, because my ECI modems give me faster throughput and bufferbloat scores of B and B and a upload of 20-30ms same as my download on thinkbroadband with no QoS on at all. I don't think the 166 is really designed for VDSL2 and the 130 has had not updates since around Oct 2019. The 130 came out in 2013 and must be heading to end of life soon.

I have a few backup ECI units built in 2016, I may grab a Vigor 130 while I can but even that needs the alternative code I've been told on ECI cabs. Since the Vigor 130 has exactly the same chipset as the ECI B Focus r2 I feel right now I would be wasting my money at this time. I'll wait to see if they cure the 166's issues but it should never have been released in this state. I understand the pandemic has caused issues, I guess they were pushed to get the new hardware out as it was already late, either way its now in its box, with myself hoping they can stop the upload spikes of 243-343ms and the packet loss. Setting the 166 MpoA on 1492 stopped the dropouts but in total I lost 10Mbps download and doubled my latency from 7ms to 14ms. No matter what's happening, its now March and no fix as of yet. Maybe its just a lousy chipset for VDSL2 but good for G.fast :?

Well they certainly haven't learnt their lesson!
I had a V166-A GEN2 on loan for a couple of days (from a mate who ordered it from Amazon in Germany) & it's just as bad.
This is probably because it shares the same firmware as GEN1 (4.1.1.STD), they haven't even bothered to use updated firmware on it
I can't believe it's a hardware problem, if the chipset performs this badly (inc. the new one in the GEN2 model) I think Intel would have noticed.
It has to be the firmware, with so many bugs evident in both the stats readouts & CLI commands (& of course the lack of ADSL support) it's obviously a bodged job.

Chipsets can cause issues and intel chipsets are not that great now although as you say it should be the firmware, its just that Intel where having issues with modems in Phones compared with Qualcomm, and could not get a 5G chip working so sold their mobile modem business to Apple, so Apple went back to Qualcomm in the end and will use Intels failed designs and rework them I guess, so everything is in house eventually, like the new Apple silicon which is out pacing intel, AMD are also doing much better than Intel too and intel have another security issue as well now 'https://www.theregister.com/2021/03/08/intel_ring_flaw/'

The intel mobile modem chips were slower in the same places and did not hold onto the signal as well which was an issue. I hope its not the chipset, but putting the ECI Openreach modem back on I'm now getting bufferbloat of A down and A up on TBB with no QoS and I have got my speed back to 53Mbps and my pings are now 7ms not 14ms, it took two days for that to happen once the Draytec was removed. I really wanted the Draytec to work but right now I'm not disturbing my line again for fear of messing things up. I'll let others be the beta testers, for once. Lets hope it does get sorted sooner rather than later though as it would be good to use the thing, right now its a expensive paper weight. I have to say apart the fact there are no stats CLI commands work fine, its just that its not showing errors at all.

I can't see the firmware causing such huge spikes in latency, right now upload is 23ms with the Vigor 166 it was 234ms which is crazy. That could be hardware but like you I'm hoping its firmware. Draytec support has been great, but they seem at a loss and I don't think Covid is helping either, they must be under a lot of pressure all round. I'm sure I read somewhere that ADSL support has been removed from this modem, so its G.Fast, VDSL2 and VDSL only and no ADSL for this one.