error when turning on with Cerys mod
Turning off nuclear reactor scaling helped to launch the game

Can you show the error message?

Error while running setup for entity prototype "100-cerys-mixed-oxide-reactor" (reactor): next_upgrade target (bob-nuclear-reactor-2) must have the same fast_replaceable_group (nuclear-reactor != mixed-oxide-reactor).

Sorry, didn't see that he wrote about the bob reactor
Without bob mods, it starts without changing the settings

This seems like bobs mod is doing something strange - I could theoretically blacklist the Cerys reactor or manually reset next_upgrade for this special case, but I think it would be better to report this in the Bobs mods discussion.

Either they set next_upgrade of the vanilla reactor before Cerys copies it as a bluepring for the mixed-oxide-reactor (which could be fixed by moving that change to data-updates or data-final-fixes), or they are changing next_upgrade for all reactor entities without looking at fast_replacable_group...