CRM 2013: Translation import error: The value passed for ConditionOperator.In is empty

This week a very evil error drove me nuts.

In CRM 2013 I tried to import a translation file into an organization.
It always ended up with this error message: The value passed for ConditionOperator.In is empty

I was like what in the world does this message mean.
If an old school approach in splitting up the file into serveral smaller Import files I finally figured out that the following lines in the "Localized Labels" sheet caused this trouble:

RibbonCustomization	11a15acc-61f1-e311-93f9-00505681252e	Mscrm.Isv.orb_input.HomepageGrid.Group0.Control0.LocLabel
RibbonCustomization	11a15acc-61f1-e311-93f9-00505681252e	Mscrm.Isv.orb_input.HomepageGrid.Group0.Control0.ToolTip.LocLabel
RibbonCustomization	11a15acc-61f1-e311-93f9-00505681252e	Mscrm.Isv.orb_input.SubGrid.Group0.Control0.LocLabel
RibbonCustomization	11a15acc-61f1-e311-93f9-00505681252e	Mscrm.Isv.orb_input.SubGrid.Group0.Control0.ToolTip.LocLabel

To resolve this problem delete the ribbon customization rows from the translation file or adjust them in the ribbon customization solution export file.

In my case this problem was that these lines came from a CRM 4.0 to CRM 2013 migration, although I do not need this customization anymore.