Don't have a solution to your problem ... but ...
Plugins page for FexPage Format @ https://moodle.org/plugins/pluginversions.php?plugin=format_flexpage doesn't show a version compatible for 3.5.x.
Besides that, with the recent changes to MoodleRooms, I see that the plugin is offered and maintained by MoodleRooms Development (see link above). Contact/Lead Maintainer info on same linked page towards the bottom.
This to say you might have to try to contact developers and inquire or change formats until the dust settles @ MR.
'spirit of sharing', Ken