Hello ,
Please open Twinmotion and try downloading a Sketchfab asset again. When you receive the message asking you to sign in, please close Twinmotion, then locate the Twinmotion.log file created from your session (Where to find Twinmotion .log file). Please submit this log file in a bug report here. Our support team can investigate the issue and let you know what might be happening.
Best,
Raghib