It is possible to see the error, Error Opening folder! Error #21 when trying to open a folder/repository from r11.5 Web Viewer. This is a different situation from the Error Opening folder! Error #6.
What does the following error message mean when attempting to logon to the Output Management Web Viewer:
Error Opening folder! Error #21
Windows 2008/2012 Server R2
Error #21 means unknown error. Output Management Web Viewer has received an unknown error from DRAS. Examine the DRAS logs for possible causes. If the problem is not clear, contact Technical Support