Hi Dieter,
There may be issue with how your website it set up on the server which does not allow Virtual Path. Any file that is referenced with a leading slash "/" will not work on your server setup.
By default the date pickers reference all the files via a virtual path.
As noted, the Url properties on the full version (BasicDatePicker) can be revised to point to the Application Root, although those properties are not available on BDPLite. but there is good news...
Our next version (v1.2) includes a ResourcePath property on both BDPLite and BasicDatePicker which allows the developer to change the path to reference the Application Root instead of the Virtual Root.
You can download our latest build of v1.2 at the following location. See http://forums.basicdatepicker.com/forums/505/ShowPost.aspx
I would highly recommend uninstalling the current version and reinstalling with the latest v1.2 build.
Hope this helps.
Thanks,
Geoffrey McGill
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Coolite Inc.
Email: support [at] coolite [dot] com
Phone: +1(888)775-5888