I had been working on this issue lately where customer was using Outlook 2003 to create custom actions. It is not very commonly used thing so you might not know about it, at least I was not very much aware about it.
To do that, you have to open a form in design mode and go to Actions tab, hit the New action button down in the bottom left corner.
Give it the name of your choice and select “Appointment” type form from “Standard Forms” template. It will look something like this..
Publish this form and send it to your self.
Once sent, it will come up like this to your inbox, see the “CustomAction” button (this is the name that you provided)
Once you click this button, it should open a new Appointment form like when you do a forward or reply button, but instead of doing that it shows an error.
Error: "Could not complete the operation. One or more parameter values are not valid"
I found this problem to be related with build number Outlook 2003 11.8313.xxxx (SP3) and it does not reproduce with the standard install of Outlook 2003 SP3 (build 11.8169.8172).
So who caused it? Looks like that a latest security update has caused this issue.
Security Update for Microsoft Office Outlook 2003 (KB973705)
I have tried to uninstall this update and it started to work just fine, installed it back and it breaks again.
Since Outlook 2003 is out of commercial support lifecycle, you may or may not get an hotfix for this. If you have a business justification and this issue is causing trouble to you then please do not hesitate to contact me.
If we ever decide to fix it, then I will update this post and update you all.
PS.: The only possible workaround, though highly not recommended, is to uninstall the security update but be aware of the risks of doing so.