Hello,
I found a somewhat reasonable workaround for the cache (re)generation problem: run apache2 as the same ispconfig's created regular user.
It is not a clean solution and do not sized all effects of it, but works for a single hosted site.
For more sites inside the ispconfig, a correct solution must be found.
Regards. Andre Felipe Machado
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A Debian user never dies. Issues a last command:
shutdown -h now