"Error Creating Control" when creating a custom control (Design Time). Can't see tooltip message.

[Ravi Ambros Wallau]

Hi:
I've created a custom control - a grid that uses Infragistics to display
some filters, the grid itself, and some buttons.
Well, when using this control directly on WebForm, everything works
really fine. No problems at all.
But, when this control is placed inside a template of an Infragistics
UltraWebTab control, somethings stranges happens:
1. When the WebForm is opened, an "Error Creating Control" message is
displayed (for container control, the UltraWebTab one), and I can't see no
detailed error message (a tooltip is not displayed);
2. If I go to HTML view and back, the control is displayed;
3. But, if I change anything on the page, the "Error Creating Control" is
displayed again... And, this time, the reference for the control is removed
from the source code, for that page.

I'm stuck. Does anybody knows how to display a detalied error message
instead of the useless "Error Creating Control"?
There's something really wrong, but I'm not a lucky man to guess what's
wrong without knowing what's happening.

Thanks,
Ravi.