In version 9.1.5 we upgraded the FlexLM license manager to from version 11.14 to 11.16 because of a potential security issue reported by a user.
This upgrade breaks client-server compatibility. That means that if you installed a MOSEK 9.1.5+ client, you are using a floating license, and you are getting an error such as
Version of vendor daemon is too old
then you should upgrade the license server to one that also comes from a 9.1.5+ distribution of MOSEK. It will be able to serve the new and all older client versions.