Euw. Looks like the character limit is 256. You can probably leave off DefaultItemOpen=1, since having Source should cause the form to open in the browser. I'm wondering if you can use relative paths for the XsnLocation, SaveLocation and Source.
http://620-650-314:100/IT/_layouts/FormServer.aspx?XsnLocation=IT/IT Activation Form/Forms/template.xsn&SaveLocation=IT/IT Activation Form&Source=IT/IT Activation Form/Forms/Open Activation Forms.aspx
May be worth a try, right?