Here’s the announcement stating all the datastore changes that are taking place soon.
What kind of string are you trying to save? I did find this:
Any string being stored in a data store must be valid UTF-8. In UTF-8, values greater than 127 are used exclusively for encoding multi-byte codepoints, so a single byte greater than 127 will not be valid UTF-8 and the GlobalDataStore:SetAsync() attempt will fail.