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Starting the latest version of the core ends up with "Illegal function call 0" in the MSX basic screen. Also it seems that the OSD for the core is missing quite many options 
Earlier versions work just fine. Is my configuration somehow funky or what?
I just tried a few different games and they all play fine on my end. Maybe your BIOS file got corrupted somehow? Try redownloading it
I had same issue so i've deleted all MSX1 config files and RBF files then i've downloaded from 0 following files:
https://github.com/MiSTer-devel/MSX1_Mi ... 3_test.rbf
(with this oldest core i've recreated a new working configuration and saved it) then:
https://github.com/MiSTer-devel/MSX1_Mi ... 2_test.rbf
and after i didn't remember if i've used this one:
https://github.com/MiSTer-devel/MSX1_Mi ... 8_test.rbf
or this one instead:
https://github.com/MiSTer-devel/MSX1_Mi ... 250108.rbf
these lastest 2 files are not same