Hi Ryan -
Thanks for looking into this, and after further testing, I have some findings. First, none of the users are using thumbnail view in Web Access, and we were not using it when testing - it is the standard folder view.
Further, we have found that if the starting entry of a business process is a document, when you log into Web Access and select the document, the Start Business Process button for that process is grayed out, however, if you do anything that refreshes the screen - click Refresh, fill a field, rename the doc, etc. - this will activate the Start Business Process button, and you can now start the business process normally.
If the starting entry of a business process is a folder, this does not work. With folders, if you select the folder in the left pane (the folders pane), the Start Business Process button is always grayed out - refreshing does not make it active; however, if you select the folder in the right pane (the contents pane), the Start Business Process button is active immediately, and no refresh is necessary.
I don't think this is a general bug, as I can't remember ever seeing it before in any other environment, but I can't be certain. At least now, I have some pretty specific conditions under which the issue is happening, so hopefully you'll be able to reproduce it or not.
Thanks for your time!