I believe you are trying the conversion using desktop application Adobe Reader DC? And the application isn't letting you to either Sign In or Sign out ?
If this is the case, then you may check whether you are able to access the service online via- https://cloud.acrobat.com/exportpdf or not. There could be a possibility that the application itself causing problem.
If its working online and you are able to convert the document, then you may try the following-
Open Adobe Reader DC>Help>Check for update
Or Open Reader> Help> Repair installation
Still no luck, then use cleaner tool to uninstall the application - Download Adobe Reader and Acrobat Cleaner Tool - Adobe Labs
And reinstall it using- Adobe Acrobat Reader DC Install for all versions
Let us know if it still doesn't help.