On attempting to add a 3rd party webpart to my Share Point 2007 installation I got the following message in a dialogue window:
Unable to add selected webparts. Cannot import this webpart
I reviewed all the instructions and I had followed every step.
One of the steps mentioned adding a safecontrol entry to the web.config.
On changing the order of where this safecontrol entry was in web.config, to the top of the list, the webpart displayed perfectly.
My thinking is, that one of my other entries is causing an error, and stopping the rest from loading…
Anyway, next time you get this error, try moving the Safecontrol entry to the front of the list, and see if it resolves the problem.