[horde] Horde only working correctly in Basic mode

[Bzzzz]

Debian Stretch + Backports + several external repos.
Personal installation as a test before going to prod.
DB: Postgresql v.11
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Hi listers,

recently I set a Horde 5 from the Stretch packages and struggled a bit to
make it work.

I finally had it to work correctly reading here and there.

The authentication process is set to:
	"Let a horde application handle authentication"
	imp

but there a big glitch, mail & calendar correctly work only in basic
mode, automatic & dynamic have the following effects:

* Imp can't connect to the IMAP server, emitting an error msg:
	"Error when connecting with the server".

* Calendar returns a full blank page.

The problem is, even in debug mode, there's no indication of any error
in DEBUG.log, so I'm a bit lost and out of options - other modes are
mandatory, as the basic one doesn't have a 3/3 e-mails display.

Jean-Yves