Any mJS code gives exception and followed by core dump on ESP8266 (release 2.20.0)

No need to downgrade the mos tool.
One can specify the version to be used in mos.yml, e.g.

libs_version: 2.19.1 #${mos.version}
modules_version: 2.19.1 #${mos.version}
mongoose_os_version: 2.19.1 #${mos.version}
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