Issue #1795: advise against using colons

as custom code identifiers.

Kernel/System/UnitTest/Helper.pm

@@ -544,6 +544,9 @@ Also note that the deployed custom code is only executed when a new instance of
 is created. This is done for every web request. But within a test script you must discard the
 config object, so that the object manager has the recreate it.
 
+It is advised that no fancy characters, like B<:>, are used in the identifier. Using B<:> in keys
+has caused errors when running with an S3 compatible storage in the backend.
+
     $Helper->CustomCodeActivate(
         Code => q^
     sub Kernel::Config::Files::ZZZZUnitTestNews::Load {} # no-op, avoid warning logs