Method 'Linked Cell' of object 'IMdcCombo' failed - help?

Using VB 6.3 - trying to use a ComboBox to populate which level is being selected. The available levels change based on 4 option buttons.

Whenever I try to select a different option button, or level in the ComboBox, I get this error message and Excel crashes:

Run-time error '-2147417848 (80010108)': Method 'Linked Cell' of object 'IMdcCombo' failed

Any help would be greatly appreciated! I don't know what I am doing wrong.

Thread moved to 'Office Development/VBA' forum (note that the "VB Editor" in Office programs is actually VBA rather than VB, so the 'VB6' forum is not really apt)