Hi Fabian, sorry for the answer after all these years - I didn't receive an email (maybe spam filter?) and had forgotten about this bug report.
> could you confirm that removing the lines you quoted from > 57-dejavu-sans.conf enables the changes in the fontconfig-config shipped > 60-latin.conf file to take effect? If this is really the case, we should > probably ask fonts-dejavu upstream to remove the offending lines from > this config file. Yes, I can confirm this. I would like to ask the upstream developers, do you know where I should do that? Thanks Jan