Unable to click "Add New" for related records. "The Tag is Invalid"

I'm having a weird error in CRM where nothing happens when I click on the Add New ... button for any entity. Note that this only happens after I have created a record. For example:

1. Create a new Account
2. Click on Save
3. Click on Contacts on the form navigation while still inside the Account form
4. Click on Add New Contact

Result: Nothing happens

If I attempt to create a new Contact from an existing Account, it works as expected. For example:

1. Open an existing Account
2. Click on Contacts on the form navigation while still inside the Account form
3. Click on Add New Contact

Result: The new Contact form is displayed

** PLEASE NOTE ** - this happens on three organisations on our deployment, including a clean instance of CRM withno customisations.

Here is the outputted error when CRM asks you if you want to send error details to Microsoft.

<CrmScriptErrorReport><ReportVersion>1.0</ReportVersion><ScriptErrorDetails><Message>The tag is invalid.</Message><Line>6</Line><URL>/_common/global.ashx?ver=1791294139</URL><PageURL>/main.aspx?etc=10062&extraqs=%3f_gridType%3d10062%26etc%3d10062%26id%3d%257b68B5C4EF-55C6-E211-8EDB-005056824F19%257d%26pagemode%3diframe%26preloadcache%3d1369608665050%26rskey%3d135650052&pagetype=entityrecord</PageURL><Function></Function><CallStack></CallStack></ScriptErrorDetails><ClientInformation><BrowserUserAgent>Mozilla/4.0 (compatible; MSIE 8.0; Windows NT 6.1; WOW64; Trident/5.0; SLCC2; .NET CLR 2.0.50727; .NET CLR 3.5.30729; .NET CLR 3.0.30729; Media Center PC 6.0; .NET4.0C; .NET4.0E; InfoPath.3)</BrowserUserAgent><BrowserLanguage>en-us</BrowserLanguage><SystemLanguage>en-us</SystemLanguage><UserLanguage>en-nz</UserLanguage><ScreenResolution>1920x1080</ScreenResolution><ClientName>Web</ClientName><ClientTime>2013-05-27T10:51:28</ClientTime></ClientInformation><ServerInformation><OrgLanguage>1033</OrgLanguage><OrgCulture>5129</OrgCulture><UserLanguage>1033</UserLanguage><UserCulture>5129</UserCulture><OrgID>{7C573D12-662F-E211-8AB0-005056824F19}</OrgID><UserID>{2161F3E6-646E-E211-B123-005056824F19}</UserID><CRMVersion>5.0.9690.2243</CRMVersion></ServerInformation></CrmScriptErrorReport>

Any help will be greatly appreciated.