Unique field found, although there is none.

Hello!

I've found the following strange issue today:

"The date field requires a unique value, and the specified value is already used. Matches are found in the following content:"

So unique field finds a match, but even it can not point it out, there is none displayed.
The field in question is a date, with only the start date collected and only year-month-day. The unique field settings does not seem to matter, it always finds a match, but only at a certain date (2015-02-10).
I've tried to make other fields unique in that content type instead, and it works. Even if I try it with another date field.
I've deleted all the other nodes in that particular content type.
No unpublished nodes in that content type.
I don't use that particular date field anywhere else.
Also I tried to add other fields to the unique field group, and use the "combination of fields to be unique" setting, but still shows this error.

Rather strange, any ideas?

Thanks!