Step 1. Confirm the usage of drive C, proceed step 2 if it's not full.
Step 2. Open the path of %appdata%\RenderBus\1005\Profiles in the computer, copy the users folder as backup files. Then delete users folder, try loging in the app. Proceed step 3 if its not working.
Step3. Report this error to the online customer service on www.renderbus.com, they will contact with the R&D department to fix it.