The error I see most often is 400 - Bad Request
I have created a zip file containing a number of replacement files. You can download the file from my Google Drive here.
Once you have downloaded the fix you can extract the contents of the IPO11Fix.zip file:
Navigate to the IP Office installation directory and find the common memory card folder (in a default installation that would be C:\Program Files (x86)\Avaya\IP Office\Manager\MemoryCards\Common\). Paste the system folder that you had copied into this folder and overwrite all the files when prompted:
You may be prompted for Administrative access. Provide the access if requested: