This problem with the LinkClick handler (and also the Captcha handler, etc.) stems from the AppPool running under ASP.Net 4.0 and the need for some minor changes to be made to web.config.  Find the following <handlers> section inside of the <system.webServer> node:

<handlers>

<add name="LogoffHandler*" path="Logoff.aspx" verb="*" type="DotNetNuke.Services.Authentication.LogOffHandler, DotNetNuke" preCondition="integratedMode,runtimeVersionv2.0" />

<add name="RSSHandler" path="RSS.aspx" verb="*" type="DotNetNuke.Services.Syndication.RssHandler, DotNetNuke" preCondition="integratedMode,runtimeVersionv2.0" />

<add name="LinkClickHandler" path="LinkClick.aspx" verb="*" type="DotNetNuke.Services.FileSystem.FileServerHandler, DotNetNuke" preCondition="integratedMode,runtimeVersionv2.0" />

<add name="CaptchaHandler" path="*.captcha.aspx" verb="*" type="DotNetNuke.UI.WebControls.CaptchaHandler, DotNetNuke" preCondition="integratedMode,runtimeVersionv2.0" />

<add name="UserProfilePageHandler" verb="*" path="User.aspx" type="DotNetNuke.Services.UserProfile.UserProfilePageHandler, DotNetNuke" preCondition="integratedMode,runtimeVersionv2.0" />

<add name="SitemapHandler" path="Sitemap.aspx" verb="*" type="DotNetNuke.Services.Sitemap.SitemapHandler, DotNetNuke" preCondition="integratedMode,runtimeVersionv2.0" />

<add name="RadProgressHandler" verb="*" path="Telerik.RadUploadProgressHandler.ashx" type="Telerik.Web.UI.Upload.RadUploadProgressHandler, Telerik.Web.UI" preCondition="integratedMode" />

<add name="Telerik.Web.UI.WebResource" verb="*" path="Telerik.Web.UI.WebResource.axd" type="Telerik.Web.UI.WebResource, Telerik.Web.UI" />

<add name="Telerik.Web.UI.ChartHttpHandler" verb="*" path="ChartImage.axd" type="Telerik.Web.UI.ChartHttpHandler, Telerik.Web.UI, Culture=neutral, PublicKeyToken=121fae78165ba3d4" />

<add name="HtmTemplateFileHandler" verb="*" path="*.htmtemplate" type="DotNetNuke.HtmlEditor.TelerikEditorProvider.HtmTemplateFileHandler, DotNetNuke.HtmlEditor.TelerikEditorProvider" preCondition="integratedMode" />

</handlers>

Whereever you see preCondition="integratedMode,runtimeVersion2.0" change it to either preCondition="integratedMode" or to preCondition="integratedMode,runtimeVersion4.0". Either value for preCondition will work - still not sure which is most appropriate. I had logged this as an issue in Gemini a while back but it was not on the top of the list of fixes for this release.

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Bill, WESNet Designs
Team Lead - DotNetNuke Gallery Module Project (Not Actively Being Developed)
Extensions Forge Projects . . .
Current: UserExport, ContentDeJour, ePrayer, DNN NewsTicker, By Invitation
Coming Soon: FRBO-For Rent By Owner

Done! You were right Bill, i recomend to evryone to change the web.config handlers to preCondition="integratedMode,runtimeVersionv4.0" .
Without runtimeVersionv4.0 some handlers hang, not sure why but after some minutes the site it becomes frozen and this it happened twice in a row, is like is never gonna finish loading and is unreachable.
It looks like once changed the preCondition there are no more errors and lots of things work normal...

Thank you DNN Team!

Teste this on ISS7 with the latest updates on ASP.NET 4 Framework. / DotNetNuke 5.5.1

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Thanks, but still i´m not clear about this issue as if i remove all ",runtimeVersionv2.0" it still breaks some modules...

I´m sorry to say this, i have two years now since i´ve first used DNN and to tell the truth i´ve learned a lot but to be honest at this time i´m looking to see if i can find another CMS as DNN is becoming with every release more and more unstable and slow.

DotNetNuke is ok and you can do so many things with it and the way it can be extended is unbelievable, but lets be honest, DNN on a shared environment is NOT RECOMENDED for an every day use.

Not sure if i explain myself very well, i´m currently running two DNN sites, one at PowerDNN and one at DiscountASP.NET... the both hosting platforms are offering almost the same performance, but the problems are the same in any direction cause both instalations are running in an isolated environment with a small/limited amount of memory and lots of restrictions.

I´ve invested a lot ($) in modules and software from the SNOWCOVERED market but this is in the end for nothing cause if you buy a module it is very possible that with the next release of the DNN something goes wrong and is more likely to find yourself again surfing the forums and the community to fix something what you think it should be working fine. Also i´m currently maintaining another installation for one of my clients on a VPS and it was a trouble to make it work at the begining but in the end with the help of the community i´ve succeded...

I´m not trying to encourage anyone to change from DNN as i find it for myself dificult at this moment to start again with another CMS but i just cannot lose any more time fixing and fixing and spending nights to make it work as it should. It´s ok to find a bug or a minor error somewhere inside your site and ofcourse report it to the community and trying to fix it all toghether, but to find that a "change" it was made by the DNN team and so your site goes down or a module that you have bought just does not work anymore, well, in my opinion this is unacceptable. :(

In the hope that maybe someone from the DNN core team will read this, in my opinion DNN is going to a wrong direction, maybe you/we should test more a new release before it is ready for a "stable" status. I´ve noticed that many developers from the SNOWCOVERED market are left behind and this is their opinion also.
Oh,yes, i know, the community edition is free, and we all thank you for this, this is the biggest community i belive, or not!?
We are here to help and get help, but think a litle bit, this are not just my words, maybe i´m speaking for the entire comunity and in my these two years with DNN i´ve seen lots of my friends moving to other CMS like umbraco or kentico or mojo... Why is that!? Well, two years ago when i´ve started with DNN the first thing i´ve found in a forum dedicated to DNN is that "IT IS NOT RECOMENDED" on a shared environment... 

...a small "begginer´s advice", maybe DNN it should begin using a more BASIC structure, at least one "dedicated" for those ho are using a shared environment cause if not, mark my words, DNN will lose ground very fast in what is going to come in the CMS market...

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