The "ViewHostUndefined" error message resource has an invalid key. There is an appended "." to the end of the key causing issues changing the value as well as serializing the value.
Tested Sitefinity 9.0.6030 as well as 10.1.6502 both broken.
I posted this in the feedback portal as well but it is currently marked as private: https://feedback.telerik.com/Project/153/Feedback/Details/229581-viewhostundefined-bad-resource-key