I’m using free eM Client with Windows 10 version 1903. For some time now eM Client usually becomes unresponsive whenever I click on the paperclip to attach a document to an e-mail that I would like to send. My only choice is to close eM Client and have it go through a complete audit before restarting. Very occasionally eM Client actually does manage to open Windows Explorer and allows me to select the documents that I want to attach - but this is very much the exception. More often than not I have to go out to my gmail account to get the document attached and sent.
Which version of eM Client are you using? (Menu > Help > About)