OK. I've found the solution to why this problem exists: The Linux server didn't have a "path" problem, especially not in limiting, but a "name" problem with certain Windows ANSI special characters in the name of files that are stored by Windows over network on a LINUX server.
I'll explain it here in as much detail as possible to help other users affected by this problem:
(1) Windows displays all special characters without problems and/or conversinon that are contained in the Windows ANSI character table.
(2) Linux has - especially not e.g. in the ext4 file system - no ANSI character table, but only an ASCII character table.
(3) In Linux, only 255 characters may be contained in a file name, too. See: https://wiki.ubuntuusers.de/Dateisystem/#Weitere-Merkmale.
(4) In Windows the file name:
19-01-02¦11.44¦ --» EINGANG Gr-AW 'Pfad wird bei jeder manu-Sicherung v Assistenten gespeichert. Habe so geändert, dass stets normale Langpfad verwendet w' bzgl. Problem 'Nr. 53-Datei nicht gefunden'»Pfadangabe 'datenserver--kanzlei-store--S9IRTS.msg
has exactly 250 characters (with Windows ANSI special characters).
(5) If you save this file under Linux, it is visible in a file manager (where at least UTF8 is set by default), also EXACTLY SO with 250 characters.
(6) BUT: This file is stored under Linux in ASCII character format. If you display THIS FILE in ASCII format, you get the following file name:
19-01-02¦11.44¦ --» EINGANG Gr-AW 'Pfad wird bei jeder manu-Sicherung v Assistenten gespeichert. Habe so geändert, dass stets normale Langpfad verwendet w' bzgl. Problem 'Nr. 53-Datei nicht gefunden'»Pfadangabe 'datenserver--kanzlei-store--S9IRTS.msg
This file name stored on the LINUX server has exactly 255 characters, so that neither LINUX nor Windows over network (where only 250 characters are displayed, see above) can add only a single character to this file name via the network.
Why?
It is probably because these Windows ANSI special characters contained in the file name do not exist in the LINUX ASCII table and LINUX AUTOMATICALLY converts these characters and places its own characters in front of them.
RESULT: Even if the file name of a file stored over network on a LINUX server contains only 250 characters in the file manager of Windows, 5 characters cannot be added to this file name, because the same file on the LINUX server already has the maximum number of 255 characters.
Well, the Windows and LINUX world is sometimes not easy (...).