V02 Max no longer recording

Sorry. no suggestions but my 945 on latest softwear version is updating VO2 max perfectly even with short run.

Have you checked your HR device? Did you use OHR or a HR belt? For VO2 calculation (running) accurate HR (and HRV) data is necessary. For a marathon a exercise load of 1857 seems extremly high. Is that correct? For comparison, just did a 90k XC skiing competition in 7 hours with around 250 in exercise load. Doing around half the exercise time with almost 10 times the exercise load seems very high but I could be wrong. Training load is based on HR data.

Check battery in your HR pod (unless using OHR) and the contact with belt to any see noise or irregular data. What does your HR graph look like?

On the other hand, 250 as exercise load for a 7 hour Vasalopp seems very low - when I run my interval workouts it's not uncommon that I have around 250 as exercise load, and then we are only talking about a sub-hour effort (albeit with high intensity).

Yes, 250 in training load for the Vasaloppet could seem low (I double checked and had 258 in TL). But I also only had around 500 in training load when running the UTMB CCC a couple months ago (a ultra trail run 101k with 6300m elevation gain in 20 hours). For the Vasalopp I took it rather easy and was most of time in zone 3. However, I also get around 150-200 in TL when doing a hard interval/threshold run. The TL model seems to undervalue duration. For me, it is tougher on the body (ie higher training stress on the body) to be running in zone 3 for 7 hours compared to a one hour interval session but the one hour interval session generates the same TL.

If I do an interval session in the morning I have no problem having a good second run in the evening. But if I do a 8 hour long run I am usually not in shape to have a good/comfortable run the day after.

Having a TL of 2000 for a 3-4h run would suggest being in Z5 the whole time which seems "impossible". Getting a TL of 5-600/hour is very high. Would be interesting to see what others have as TL for longer duration events (when HR usually is a bit lower). Training load only seems to start accumulating around HR Z3. Last autumn I had a long mounting hike (6 hours) with a heavy backpack in Z1/Z2 which gave me a TL of 6.  

I think I saw something from Garmin some time ago that "professionals" have a weekly training load of around 1100.

From Garmin: "If you are looking at extreme training loads resulting from a single workout, you might see top endurance athletes with numbers occasionally in the 300-400 range."

https://www.garmin.com/en-US/blog/fitness/ask-expert-training-load/