I've got this warning message from Biblatex, after several runs, this message still appears.
Since my bib-File has about 100 entries, and I cannot find any failure, I'm wondering which behaviours could lead to such an error?
Every entry has an "keywords" defined, and I'm ordering the bibliography by keywords. I just saw this error today and I don't know when it arose the first time. So I'm interested in a list of points to check, how this error could occure. I run the py-parser for bibtex  on it, but it doesn't show me up the error.
I copied all of my preamble into a new file and copied the bib-printing part, too. The result is well printed. So my question can be reduced to:
What mistakes could I do in the content part, that leads to this warning and a bibliography with "" in every entry, instead of its real entry.
One mentioned, that I could hit in a misspelling or a missing link. Are there any other failure sources?
My experiments have shown, the most efficient way to get such a behaviour is to use any cite command with an undefined reference, even if it is only a typo or really not defined. (Well, biblatex makes exactly what is intend to do: if a string is not an entry, it arises an error. So even typos are not defined entries ;) )
If someone ends up into such a problem just reduce your problem to the most smallest one. That means: First: Check if any "keywords" is set, if used. Then: Use your whole preamble and print the complete bibliography. If this ends up in the same error, your problem might be in your preamble. -> Reduction of preamble, until you get your bib printed. Adding slowly parts back to the preamble, until the error occurs again. That might show you, what lead to the warning.
Otherwise input small parts until the misbehaviour appears. -> Reduction of the part, until your problem does not appear anymore.
Your way is, therefore, a reproducing way. Setup the "backend", include steps of the "frontend" (content) and look for the file, that leads to the error.
 https://github.com/Pezmc/BibLatex-Check <-- this is only the used checking software