My widgets in panel and system monitor render only text instead e.g. pie chart.
What's more, System Monitor states that it "has fallen back to software rendering because hardware acceleration is not available ...".

What causes this behaviour? I have accelerated OpenGL and I recall those widgets rendered some time back (really do not recall when).

Some more information about your system might be useful if you want an answer, like your distribution, exact Plasma version, graphical setup, etc.
You can find all this information in System Settings -> System Administration -> About this System