Have had Solid Explorer since the "Half Price" sale and used the free one before that. Worked great till the recent Android Version update to Oreo. I NEVER assigned a "Master Password. PERIOD. Only used the fingerprint. worked flawlessly. After the up date it started asking for my password on cloud storage. Fingerprint does not work, and just as you say, asking for a reset blocks all access ( that you did not have anyway) to the cloud. It also uses the incorrect username - my internet mail username and not my Google account username, ????
Tried a complete uninstall - reinstall, deleting cache and data storage -no luck'

If you cant fix it at least tell me the name of a good File explorer. Or do have to buy a new Cell?