Strange recurring line in errorlog

[Reko Turja <reko.turja@xxxxxxxxxxx>]

Dear Apache experts;

I have a recurring line appearing in my error log:

Host: not found

Now and then the line appears every few seconds and then it might not
appear in a good while. Strange thing is, that there is no [tag] or
timestamp or anything else attached.

Server is FBSD 10.3 running 2.4.latest with PHP 5.6.latest in php-fpm 
mode.
Otherwise the modules etc. are pretty standard and minimal.

Any clues for finding the root cause for the line would be nice.

Reko Turja

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