Users of my site upload images. Once or twice a day I get this error...

Error in page :https://photocompentries.net/eo_web.ashx?id=4EF2A06E-5171-4166-82A6-242BD1985E8A&uploader_data=ByF1cGxvYWQCgK2Aq2N0bDAwX2NwTWFpbkNvbnRlbnRfVXBsb2FkZXJfMjpJNlBkR0g4RHFld1Vod3FZNndDVEE2VHNFNFlLb3VnSmtRK240UW1XOTRXNzk0TGZsdXdGZis2czJSS1dBSmJVOTVyOG8rc0poUGVKM1JHVDVLTFpDNGdPc2FoNHNnQnNDWnJzRkpqOWtlNE1rZzZwNndDVEE2VHNFNFlLb3VnSmtRK240UW1XeVE9PScmczo3ZGNmMmI5Yi1hZDY5LTQ0M2MtOWJlMC1mYzE0Y2RjOTcyMzU=
Message
The input is not a valid Base-64 string as it contains a non-base 64 character, more than two padding characters, or an illegal character among the padding characters.
StackTrace
at EO.Internal.gswx.nfgk(gsyi fjr, HttpResponse fjs, Object fjt) at EO.Internal.gsyk.ecdb(gsyj das, Boolean dat, AsyncCallback dau, gsyi dav, HttpResponse daw, Object dax) at EO.Internal.gsyk.nfgk(gsyi dap, Boolean daq, AsyncCallback dar) at EO.Internal.gsyk.eccz(HttpApplication dal, AsyncCallback dam) at EO.Web.Runtime.System.Web.IHttpAsyncHandler.BeginProcessRequest(HttpContext context, AsyncCallback cb, Object extraData) at System.Web.HttpApplication.CallHandlerExecutionStep.System.Web.HttpApplication.IExecutionStep.Execute() at System.Web.HttpApplication.ExecuteStepImpl(IExecutionStep step) at System.Web.HttpApplication.ExecuteStep(IExecutionStep step, Boolean& completedSynchronously)
Query String
id=4EF2A06E-5171-4166-82A6-242BD1985E8A&uploader_data=ByF1cGxvYWQCgK2Aq2N0bDAwX2NwTWFpbkNvbnRlbnRfVXBsb2FkZXJfMjpJNlBkR0g4RHFld1Vod3FZNndDVEE2VHNFNFlLb3VnSmtRK240UW1XOTRXNzk0TGZsdXdGZis2czJSS1dBSmJVOTVyOG8rc0poUGVKM1JHVDVLTFpDNGdPc2FoNHNnQnNDWnJzRkpqOWtlNE1rZzZwNndDVEE2VHNFNFlLb3VnSmtRK240UW1XeVE9PScmczo3ZGNmMmI5Yi1hZDY5LTQ0M2MtOWJlMC1mYzE0Y2RjOTcyMzU%3d

I have set the max request length and max parameter length in web.config to 2000 and 1000.

I get a hacking attempt most days. Is this an other attemt to expoit IO web?