Symptoms

Exchange Online plug-in unavailable. Please try again when loading is complete. (Unknown (0))
Error is reported immediately after starting EWS backup after hotfix#18 was applied

Cause

During the hotfix application, the Exchange plugin (AsigraExchangeOnline_BLL.dll) was not applied /registered successfully.

Workaround

Execute the following batch files (pluginunregist.bat & pluginregist.bat) manually from command line.
Copy old dll files and (pluginunregist.bat & pluginregist.bat) in a folder and then open the commandline (run as administrator), change directory to folder where old plugin files are copied and execute the pluginunregister.bat first.
After that from command line (run as administrator), change directory to ds-client installation folder and run pluginregist.bat file.