Crippling "server is busy" errors on boot, can't find culprit process

Hello !

It's been a while now that my Windows 7 computer gets a crippling "server is busy" error.
My problem, in a nutshell : I don't manage to identify WHICH process is responsible for this, I also don't know if recognizable patterns are logged as events, or not

Description of the problem:
By crippling, I mean:
- I cannot run any program requiring to be elevated to admin level. For instance, I can start the tasks manager, but if I click the button to display the processes of all the users, the tasks manager will suddenly freeze. Or I can't also open any of the Windows administrative tools.
- Some other programs cannot run either (meaning : I click their icon or win-R their name, they exist as processes shown in the process manager, but their windows are nowhere to be seen, even with alt-tab), like Photoshop or mspaint (hence : no screenshots of the problem) and, frequently, I may click the Start button and have its entries shown, but clicking inside will be ignored
- fundamental programs don't start at all, like the antivirus (Comodo), firewall (Avast), Samsung Magician.
- I just noticed, it's an edit : opening folders, opening a library, opening the files browser, it all fails with a big white cross over a red circle, telling the remote procedure call has failed O_o

This happens most of the time on the first boot of the day, but there are days (like here, sigh) where even two reboots later, the problem is still present. It's raising in frequency, healthy boot-ups are becoming scarcer.

How it's happening to me ? Well, I start my computer, windows loads up, I can log in and even start most activities, but I'll notice some programs didn't start (antivirus, firewall, samsung magician and its annoying wish to start in windowed mode over the desktop despite /autohide)...
And after a minute or two a dialogue window will appear to tell "the server is busy", and offer to switch back to the program... except that it doesn't switch back to anything at all.

Sometimes, that dialogue window will have no icon.
Sometimes, that dialogue window will have the Avast icon.

How I tried (and failed) to fix it:
I googled the error, and found that, apparently, that's because of a program trying to access the internet too early, before the system is ready.
I thought "sure, let's unplug my router and only plug it back after windows has finished booting". That didn't change anything, same problem.

I switched plenty of services to delayed start: no change.
In msconfig, I removed from startup all programs I deemed as non-essential: no change.
I turned off some settings like time.windows.com synchronization: no change.
Maybe it's some hidden malware breaking things and trying to call home, I have Avast, Comodo, Malwarebytes (paid version: live protection) and I am generally careful, but who can tell if there aren't hidden pests, go figure.
I'm at my wits' end.

And so, here I am, hoping you may help :)

Please, would you know...
- if "server is busy" error messages would receive a recognizable entry in the events log, so that I may investigate it ? I searched and couldn't find something apparently meaningful, but maybe it's not logged as a critical or severe error, and then it's the tree lost in the forest
- if there is a way to avoid this problem, to fix it even though I can't trace its origin, perhaps ?

Thank you VERY MUCH if you can help, I'll be grateful :)

EDIT : sixth reboot of the day, still can't get things running, this time it's serious
EDIT2: the 7th reboot worked. OK for hybernation, but no rebooting anymore until I had a lead