We're running WLM 2012 on a Win 10 system which has been upgraded from Win 7 Home Premium, but I don't believe this is particularly relevant. This problem is mentioned many times in this forum and also across the web but nowhere does there seem to be an answer other than moving to another mail client (a reinstall of WLM following deletion of its databases would have the same effect of course, but the smart alecs don't seem to realize that!) 

Anyway - WLM can receive emails as usual (using a *** Email address is removed for privacy *** id) with no problems and the mails can be read using the standard setup of the WLM client, so no issue with opening emails there. However having selected an email, or having opened it in a separate window, when one clicks on reply or forward one gets the unhelpful messages which say that a problem has occurred and WLM is unable to open the email (despite it having already done so). Neither of these two messages is accompanied by any error code. We have tried the repair of the messagestore by using the esentutl.exe command as detailed elsewhere but this just runs successfully and nothing changes. 

The only thing that does look slightly unusual is that in the Security tab of the Properties window for the C:UsersXXXXAppDataLocalMicrosoftWindows Live Mail directory we see System, XXXX and Administrators with all permissions apart from Special ticked, but we also see a new User which is called "ALL APPLICATION PACKAGES" and this has just the Write permission granted. We do not see this on other Win 10 systems with WLM 2012 installed, and some limited web research seems to imply this is a Win 8 related user even though we have not come via the Win 8 route. We are not sure why this "user" is there, or whether if we were to upgrade its permissions for the WLM directory whether it would resolve the problem.  Any thoughts or clarifying information anyone?