don't know what is going on but check the permissions on the folder where you are trying to expert to. Make sure you have write permissions there. Also, first thing to try is a reboot of your computer as it sounds like perhaps something is messed up in the operating system.
oh and if you use a creative cloud version of Lightroom it could also be that the logon to the cloud is messed up. Logging out and logging back in from preferences in the creative cloud app will fix that. Due to the release of Lightroom CC it appears that adobe's servers have been overwhelmed a bit And many people have strange problems that are solved by logging out and back in.