Import Solution fails due to missing OptionSet

I've got the following error by importing a managed Solution:

ErrorCode: 0x80040203 

Specified argument was out of the range of valid values. Parameter name: 
Picklist option with value (1) that has the parent OptionSet with 
(2286e9b4-f2f9-46da-a860-aca898aa6ef7) id does not exist. 
Default picklist value has to be one of the option values.

What i did was some translations, Form changes on account and contact, and a new dialog.
The name in OptionSetView is company_language, so i think it have something to do with my translations.

Anyone an idea whats the Problem is?

Yes, the issue is that you have some Entity referencing an OptionSet and using a Default Value for the OptionSet that no longer exists.

For example, let us imagine you had Race on Contact and that you used to default Race to 'Unknown' and then you deleted the Race 'Unknown' from the OptionSet without changing the default on Contact.

You would wind up with this error. It doesn't tell you either the offending Entity or the OptionSet.

Let me know if you need more information ;-) I remember tackling this on our project a few months ago... ah, the joys of shared customization development projects =D