You may try few things to fix this issue.
1. Sign-out & Sign-in from the creative cloud desktop app for refreshing the credential authentication.
2. Reset Lightroom preferences, refer: Preference file and other file locations | Lightroom CC and 6
3. Turn off the Graphics Processor from the Lightroom preferences.
Open Lightroom > Edit menu > Preferences > Performance > Uncheck the GPU > Restart Lightroom.
Let us know if that helps.