having the same issue, in the first months i was able to connect it to my pc, but then that changed to the worse. i didnt have the time and energy to do a factory reset as the support suggested so far.
what i came up with as a workaround to still be able to transfer files onto my fp5 is quickshare which is part of android and preinstalled on all fairphones i assume..
its quite straighforward after installing the client for windows(there seem to be some ports for linux on github) and doesnt require signing up anywhere. you just have to make your phone visible for anyone and then ensure that the pin matches. if youre wifi is no too old fashioned, you can get decent transfer speeds (up to 20mb/s if i recollect correctly). thats why im certain that the data is only transferred on your local network.