Write down all account information or take screen shots. You'll need to reenter the information later. Quit Mail. Hold down the option key while using the Finder "Go to Folder" command and type ~/Library/Containers/com.apple.mail. Move the entire container, not just the contents, to the desktop. Open Mail to see if it works. Reset all account information and test. If no change, put the container back. If Mail works properly, delete the container. You may/will lose some emails if you end up deleting the container.
Do a backup, using either Time Machine or a cloning program, to ensure files/data can be recovered. Two backups are better than one.
Try setting up another admin user account to see if the same problem continues. If Back-to-My Mac is selected in System Preferences, the Guest account will not work. The intent is to see if it is specific to one account or a system wide problem. This account can be deleted later.
If the problem is still there, try booting into the Safe Mode. Shut down the computer and then power it back up. Immediately after hearing the startup chime, hold down the shift key and continue to hold it until the gray Apple icon and a progress bar appear. The boot up is significantly slower than normal. This will reset some caches, forces a directory check, and disables all startup and login items, among other things. If the system operates normally, there may be 3rd party applications which are causing a problem. Try deleting/disabling the third party applications after a restart. For each disable/delete, you will need to restart if you don't do them all at once.
Thanks for the response.
I tried moving the mail folder - and it wasn't the cause.
I did confirm that it is only my account that is having issues though, my wife and my daughter's are both fine.
It seems to be any program that uses the built in finder to attach files (including open and/or save them). I'm not sure that makes sense - but I'm not sure how else to describe it.
I'm going to go thru the Safe Mode this coming weekend and see if I can figure out what may be causing it.