Odd.
The only difference I see is that the first is a subpage and the others are root pages. I suspect that may be related to the issue somehow — possible the #salesforce_w2l_lead_4 is throwing off your theme somehow? Hard to say without digging deeper than I can without a login/ftp.
Anyways, it’s most likely a theme or plugin conflict/issue — not an issue with the Salesforce plugin — the 404 is happening long before the plugin is even invoked.
So all I can really do is:
Offer premium support (consulting):
https://thoughtrefinery.com/plugins/support/?plugin=salesforce-wordpress-to-lead
Suggest the standard troubleshooting procedure:
1. Check for theme conflicts:
Switch to a bundled theme (2011, 2012, 2013, 2014). Did that solve the problem?
If so, your theme is breaking the plugin. Talk to the theme vendor or request premium support to have me troubleshoot the issue with your theme.
2. Check for plugin conflicts:
Deactivate all other plugins: Did that solve the problem?
If not, it’s an issue with your hosting:
Talk to your host or request premium support to have me troubleshoot the issue with your server.
If it did, it’s a plugin conflict:
Reactivate plugins one at a time until the problem returns. The last plugin activated is the culprit. Report back with that plugin name/link and I can troubleshoot the issue to determine if there’s a fix.