Ok. Apart from the backup/restore issues that depend on having a healthy ext partition as I figure, the "can't mount [..] error mounting /sd-ext/!" problem is responsible for the "green HTC logo on the white screen freeze" also. So I've been trying to make it work all evening and this is what I found:
The Error (at least in my case) and the freaking "green HTC logo" freezez were generated because the Ext partition got corrupted in some bizarre and mysterious way. I have no idea how that could have happened, but I guess a small role in this ballet got played by the ROM itself and maybe even the nandroid backup built into the CWM (ClockWorkMod). It started right after I finished installing a fresh copy of the LeeDroid 2.4.1 and all my favourite apps (which worked flawlessly), rebooted into bootloader > CWM > nandroid and made the full backup of the system. Rebooting the device afterwards got me into the above mentioned freeze.
I tried many things, among which were flashing different ROMs and Radio images (without wiping though), but none worked. I just wanted to roll back to the previous state (because i've spent quite some time configuring and installing the 2.4.1) and it seemed impossible at that point. Browsing the recovery options got me into the Mount and storage section where I noticed the listed error with the ext partition. I thought maybe formatting will help - well, it didn't. After a couple of weird things I had tried, I've decided I should delete the ext partition (leaving just the fat32 intact). Creating a new ext partition from scratch seemed to solve the error. However this didn't solve the freezing. And the reason had to be the fact that when it created the nandroid backup images, the ext was already corrupt (or however unreadable), so the relative image was not written because it didn't see the source.
I doubt there is any way to fully restore my previously installed ROM with all of it's apps in place and in a functioning order, because the ext partition had data on it and now it's missing (hence the freeze).
Therefore the question: what could have gone so badly wrong to compromise the ext partition?
And another one: is the LeeDroid 2.4.1 ROM or the 126.96.36.199 ClockworkMod Recovery the culprit?
Sorry for the looong post. I look further to hearing from Lee, if his busy schedule allows it and thank him in advance.