[Exim] exim 4.22 PANICs on list expansion

Dear all -

Sorry to trouble you, but the problem's a bit urgent and I'd be grateful
for any quick fixes that you can supply.

We've just upgraded from 3.33 to 4.22 (yes, I know what you're thinking)
and all has gone not too badly.

We used the conversion script to upgrade our configure file and have
made some alterations to the latter to take advantage of the new
features. However, we haven't touched any of our directors/routers, and
exim is now throwing a wobbly when it comes to expanding an alias which
expands to a list of addresses included in an external file.

The router in question is:

system_aliases:
driver = redirect
allow_defer
allow_fail
data = ${lookup{$local_part}lsearch{/etc/aliases}}
file_transport = address_file
group = other
pipe_transport = address_pipe
retry_use_local_part
user = daemon

Debugging an attempt to deliver to one of the list aliases defined
therein (sug.comm) produces:

[snipped]

internal_search_find: file="/etc/aliases"
type=lsearch key="sug.comm"
file lookup required for sug.comm
in /etc/aliases
lookup yielded: :include:/lists/sug.comm.lis
expanded: :include:/lists/sug.comm.lis
LOG: MAIN PANIC DIE
unable to set gid=1 or uid=1 (euid=20467): system_aliases router
(recipient is sug.comm@???)
search_tidyup called

[snipped]

... which it never did before.

I note from the documentation that exim now handles it's abdication of
root priviledges differently, but I'm not sure what, if anything, this
has to do with the above.

Any assistance given will be gratefully received.

Many TIA.

randy