... solved for a part
I unistalled WA_eCart 2.1.0, downloaded the newer version WAeCart220.mxp and installed it. But then I had the error "Dreamweaver CS4 doesn't work anymore". So, I unistalled eCart 2.2.0 again and reinstalled eCart 2.1.0 back.
I also copied the file "WATrigger.js" to the folder that te error mentioned. And yes ... errors are gone, so that's already solved !
But now I get another JS-error when I start up Dreamweaver : RSSUpdate.js ... (see attach). Could it be that it has to do with the installation of eCart 2.2.0 ? Because the option "WebAssist Preferences" isn't available in eCart 2.1.0. Also when go to "Edit" and click on the "WA Preferences" I get another JS-error, see attach.
I think that some settings from eCart 2.2.0 were not removed from my pc (register, ...) and that this causes the errors !?
But how do I get rid of them ?

