Ports with unusual port names can't be opened with Qt 5.9. They could be opened with Qt 5.3, so this is a regression. If I try to open such a port, I get a DeviceNotFoundError (1).

Examples for those port names are ports created by the virtual serial port creator com0com which bear names like CNCA0 or CNCB3.

I suspect the problem with the change associated with task QTBUG-38639, where the translation of port names has been changed. Especially the commit ab51ad6a looks suspicious to me, as the translation from and to system location has been changed dramatically. Before this commit, translation was prepending or removing the default path prefix. After this commit, the assumption is made that friendly port names that don't start with the literal "COM" are alredy system locations. This is not true as seen with the CNCA0 ports. They need to be prepended with the usual prefix ("\\.\") as well.

If my assumptions are correct, the change must have happened between 5.4 and 5.5.

This behavoir can be reproduced with the example "blockingmaster", which lists, but can't connect to these ports with unusual names.