To restore your files you should copy renter.json, hostdb.json and all other files that end with ".sia" from the renter/ directory in your old sia installation to the new one. Then your files should show up in the client again.
Hard to diagnose without the full error message -- the next release should log these error messages in their entirety somewhere. My best guess is that the Rename operation is failing when the .sia files is saved. Check your renter folder for .sia files.