Ok, so what I did was that I wanted to use FrontPage 2003 to copy and paste a couple of Web Parts from one page to another in the same WSS site. Now, I’ve done this before and it has worked. But then again, it’s not that uncommon that things sometimes work and sometimes they don’t when you’re using FrontPage with WSS…
Anyway, this time no matter what I tried all I got in my browser was an error message saying Web Part Error: Unknown Server Tag.

I don’t know exactly what went wrong or if I tried doing something unsupported (as I said, it has worked before) but anyway here is the solution as I found it (which is probably the usual, proper way to do it):

1. From the page where you have the web part working export it to a .dwp file (Modify Shared Page and then from the Web Part dropdown menu select Export)

2. Import it to the other page where you want to add the web part (Modify Shared Page – Add Web Parts – Import)

This worked just fine for me. I did get a couple of error messages along the way directing me to the Web Part Maintenance Page, but this was probably because I left the non-working web parts on my page and once I deleted them using the Web Part Maintenance Page it worked fine.