Patrick;

We logged very similiar issues 7 times  with the same BC30456 alphanumeric message
The 3 listed below were fixed in release cycles:
http://support.dotnetnuke.com/issue/ViewIssue.aspx?id=11874&PROJID=2
http://support.dotnetnuke.com/issue/ViewIssue.aspx?id=7603&PROJID=2
http://support.dotnetnuke.com/issue/ViewIssue.aspx?id=4108&PROJID=2

BC30456 seems to be a generic error code catchall for aspx errors.

Did you create a new template with the same name and appended something like 2 or _2 at the end? Eg. default2 or default_2?
This thread may hold a clue: http://forums.asp.net/t/1000979.aspx this is the only Google result I could find that matched the error message you received resulting in "<  > is not a member of "

Also, would you please log this as a bug in Gemini: http://support.dotnetnuke.com/
specifically say what you named the new template(s) in the bug.

Thank you,

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Ken Grierson
DotNetNuke Corporation
Test Lead