$FILESDIR/postinstall-en-2.txt says ... 4. Read /usr/share/doc/${PN}-${PVR}/INSTALL.gz , pp. 9-12, ... but that files doesn't exists, because the correct one has the .bz2 extension (dodoc uses the bzip2 compression by default).
Created attachment 194801 [details, diff] otrs-2.3.3_postinstall.fix.patch Trivial patch to fix the reported error.
The actual compression format depends on the PORTAGE_COMPRESS variable. bzip2 may be the default, still that does not mean that every system uses it. I reworded the sentence according to the suggestion in the devmanual. Thanks.