RKCodec is specifically for Rockchip boxes and should always be considered experimental, so may well at times exhibit issues.
Adjust display refresh rate may require a delay, which will be variable because this will be the time that it takes to make the refresh rate change and the variable is that is will use different times on each persons TV set.
When looking at errors you should always consider that a new skin or addons may be the cause of some of them, so always best to ask the specific developer for advice too.
FTMC is based on Kodi 16, so I'm not sure how it can relate specifically to 17. That is not to say that there are many aspects of 16 that will have been carried forward to 17 but if a skin is specific to 17 then it's installation will likely fail because upon installation it will fail to meet the required dependencies required by an older version of Kodi.
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