After update from Kernel 5.3.18-59.37 to 5.3.18-150300.59.43 the system shows
boot messages but afterwards instead showing desktop manager login the screen
turns black. There is no signal at all (no "HDMI signal found message" on the
monitor).  Kernel 5.3.18-59.40 is also okay / not affected.

This is a rather old Core 2 Quad system with Radeon graphics which had no
similar problem until now. I found at least one similar looking Bug, please
also check if the bugfix has been merged into the latest kernel
Bug 1177384 ?

My system:
Radeon HD 6770
Driver: radeon