The problem is a limitation in Splunk's core dashboard layout system, compounded by a bug in Splunk's error messaging (See below for more technical details)
The only solution is to rework your dashboard so that you only have 3 columns of panels. In other words you can't have any col4, col5, col6 as above.
What you might want to do, is to use the weird layoutPanel values that are internal to a given rounded-corner panel. each panel, say panel_row1_col1, can have modules rendered into panel_row1_col1_grp1, panel_row1_col1_grp2, panel_row1_col1_grp3, and so on and so forth. You should be able to do 6, although at some point they may start wrapping when the browser window is not wide enough.
Aforementioned More Technical Details
As of 6.0, that error message is displayed whenever various dashboard layout limitations are hit. One of those limitations is that there can only be 50 rows. However the same message is displayed if you have too many columns, or if you have a mistake in a layoutPanel attribute, or several other kinds of problems. It's quite confusing for the customer, because of course they take the error message at face value, when most of the time it is incorrect.
I filed this with splunk support late 2013 but as far as I know a bug was never filed. (support case number was 146260).
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