DNN Module Settings causes: "Could not load file or assembly 'DotNetNuke.HttpModules' or one of its dependencies"

I am building a new portal on an existing DNN web server. I have added 20 or so pages, then all of a sudden, on some pages, whenever I try to edit the Module Settings, the browser returns the following error, even though I can do this as normal on the other 95% of pages:-

Server Error in '/Jewellery' Application.
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Configuration Error
Description: An error occurred during the processing of a configuration file required to service this request. Please review the specific error details below and modify your configuration file appropriately.

Parser Error Message: Could not load file or assembly 'DotNetNuke.HttpModules' or one of its dependencies. The system cannot find the file specified. (C:\Inetpub\wwwroot\DotNetNuke\web.config line 142)

Source Error:


Line 140:    <httpModules>
Line 141:      <!-- add name="Authentication" type="DotNetNuke.HttpModules.AuthenticationModule, DotNetNuke.HttpModules.Authentication" /> -->
Line 142:      <add name="Compression" type="DotNetNuke.HttpModules.Compression.CompressionModule, DotNetNuke.HttpModules" />
Line 143:      <add name="RequestFilter" type="DotNetNuke.HttpModules.RequestFilter.RequestFilterModule, DotNetNuke.HttpModules" />
Line 144:      <add name="UrlRewrite" type="DotNetNuke.HttpModules.UrlRewriteModule, DotNetNuke.HttpModules" />
 

Source File: C:\Inetpub\wwwroot\DotNetNuke\web.config    Line: 142


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Version Information: Microsoft .NET Framework Version:2.0.50727.3603; ASP.NET Version:2.0.50727.3082

On the page that gives this error, I cannot add a new page, otherwise I get the same error, but I can add a page from another page. I deleted one of the offending pages, and recreated it, and it started working properly from that page. Tried on another page that was failing, and it still gives the error.

The file DotNetNuke.HttpModules.dll exists under the the DNN/Bin folder and file permission of the DNN folders appears correct. No other portal on this web server demonstrates this problem.