Broken cooperation of debian mini.iso with local repository

I maintain our isolated network for specific tasks. For easy install debian system, choosing and adding the additional packages we have an easy configurable lighttpd- web server with full debian repository mirror. I download the mirror once per year with apt-mirror, single architecture amd64, testing/stable/oldstable, and the corresponding netinstall mini.iso onto a USB externeal HD. Then I copy all downloaded files to our web server, and also reconfigure files of the BootP server, to use the new mini.iso images. Additionaly I use to prepare a mobile repository of stable version on a USB flash disk for the Linux notebooks which are used off the office (deb file:///path/debian stable main contrib). Everything worked fine, but this year I cannot use the new local repository for debian installation using **netinst** mini.iso, and the mobile repository also cannot be used. The formerly installed Linux can use the new local mirror using http protocol. I cannot determine the main reason, because the netinst mini.iso just says „your mirror does not work“ but there are no logs, no details. The communication with the mobile repo indicates that there may be some problem with GPG. But the error/warninig messages also say that apt-get update cannot download the Release and InRelease files, even that these files do exist in the announced path. Well, I can accept the trend for better security on the Internet, but I would like to be either supported with valuable (optimally illustrated with examples) information how to kiss the local repository to wake it up, or how to switch off the security barriers for the fundamentally secure environment, as the isolated net or file connected repo definitely have to be. Do you know the certain way how to and what to repair in ought to be able to regain the PXE netinstalation from an isolated debian repository? After some experiments: We found, that one error comes from the multi-arch repository: I downloaded amd64 and i386. The client asks for Contents-all file, but the apt-mirror did not mirrored it. We downloaded these files manually, and some errors dismissed. But some other errors persist. We tried to create new GPG signatures, but no effect. The other suspected source of errors may be the mini.iso. It can content some config, in which we can remove the GPG requests, or add some trusted gpg keys. But even so small, it is not well documented. We can try to ask the maintainers for help.