A few of our clients are getting this error within outlook 2010.
Error while archiving folder "IPM_SUBTREE" in store "Mailbox - John Doe". The file\\server\share\archive.pstcannot be opened.
What is the best course of action to rectify the error?

Are the .pst files are located in any network share when you repro the issue? If yes, don't do it - it's not the recommended approach. PST files were never designed to be a network based solution. 

You can refer: http://support.microsoft.com/en-us/kb/297019 and http://blogs.technet.com/b/askperf/archive/2007/01/21/network-stored-pst-files-don-t-do-it.aspx

You need to make the following steps...
Step 1: Exit Outlook and start the Inbox Repair tool 
Step 2: Repair the .pst file 
Step 3: Recover repaired items to a new .pst file
Step 4: In case Microsoft tool wasn't assist you, then refer to third party software

Outlook Repair Toolbox http://www.pst.repairtoolbox.com/how-to-repair-pst-file.html

• Edited by JoseMontserrat 10 hours 46 minutes ago