Unfortunately, this isn't the only cause of the error. Sometimes a JavaScript error outside of the applet can cause the applet to fail and throw this error. However, it's most likely a typo. Reference to an image in a param tag that is mispelled. Spaces in file names.
Fixes Read through the applet tag looking for any spelling errors or syntax oversights.
Make sure to take non-essential parameters out first. Items such as desc, desturl, loadwhere and onsbtext can be taken out first. Also, please include the reason why you're sending it.