WebResource.axd requested without parameters - This is an invalid webresource request

That definitely is a bot not doing very good job of crawling your web site. It processes your web form and locates reference to WebResource.axd, for example:

<script src="/site/WebResource.axd?d=MtIW_TBRtZCvAXDMJGwg4g2&t=633772897740666651" type="text/javascript"></script>

The bot expects static JavaScript files only and tries to download it by requesting WebResource.axd without parameters. The result is an exception thrown by System.Web.Handlers.AssemblyResourceLoader class and intercepted by Application_Error in Global.asax.

I believe this exception is harmless - the client will receive 404 error. You can safely ignore it.

We also have all of our errors emailed to us, and we occasionally get those too. They never seem to have a referrer, and the user agent is usually a little wacky. We write them off as bots.

I just checked a couple of the offending client IP's against Arin, and one them belonged to a web-spidering-type organization, so there's a little more evidence for the bot theory.

I would also log the useragent that made the request to WebResource.axd. It wouldn't surprise me if it was a bot crawling your site.