Thank you gentlemen for all the support!
Looks like slowly but surely I make some progress. Just found that U2420D buffer is sinking 11mA. I measure at the U2420D output -8.8V but TP2421 is -1.25V. 11mA flowing through R2220 (680 Ohm). I've looked at the schematics and 11mA doesn't seem to be the current to be pulled out of TP2421. There are quite a bit of loads, but all of them in a range of dozens of KOhms. Something is pulling up -1.25V rail.
Also noticed, that the buffer was quite warm. So, I've replaced it by TL084N from TI. Same story, but the level changed by 10mV.
Have you observed this on your scopes when repairing?
One more thing - I've got -04 FW version. It seems like the last version was -09 for 2467 o-scope. I wonder whether I can run the latest FW. If so, then in the guts of the o-scope there should be a HW identification register somewhere to tell to the FW whether it's 2445/55/65 or 67, IMHO. Otherwise the code is almost the same.