The error pops-up when attempting to create an "E-Mail Messages" backup set (using EWS), after entering the network credentials.
If the debugging switch is enabled (DebugMessage set to 2 in DS-User > Setup > Configuration > Advanced > Miscellaneous), the following error is reported in the debug log (dsclient.exe.dbg.log visible in DS-Client installation directory, it can be opened with Notepad):
"Incompatible FtsConvert.dll (184.108.40.206,220.127.116.11) library version"
The Office 365 plug-in (v.18.104.22.168) has been installed after the Hot fix 1 (or higher) for Windows DS-Client v.13.1 was deployed. If a hot fix contains an update for the plug-in, it will not place the corresponding files in the DS-Client installation directory if it detects that no plug-in is installed.
Re-apply the current DS-Client hot fix so that the plug-in files are updated in the DS-Client installation directory.