I am just adding my feedback regarding this issue:
(Feedback has already been given in SDL community and also to the developer Mr. Gerhard Kordmann)
Step 1.: Copying the log4net.dll file did not help resolve the error.
Step 2: replacing the original folder with the contents of beta version of glossary converter.
I am able to use the glossary converter (beta) successfully.
All these steps DID NOT help to run the regular version of Glossary converter 5.1.6860.
Thank you always for your prompt guidance and help in this forum and I am always thankful so much to you!
Have a nice day!