Error when running loo with moment_match = TRUE on rstan model from previous session

Hi,

I'm trying to run loo with moment matching on an rstan model fit in a previous session. The stanfit object was saved using:

fit <- stan(...)
saveRDS(fit,...)

And then loaded and loo (from https://github.com/stan-dev/rstan/blob/develop/rstan/rstan/R/loo.R) was run with:

fit <- loadRDS(...)
loo(fit, moment_match = TRUE)

which throws the following error:

Error in .local(object, ...) : 
  the model object is not created or not valid
Calls: loo ... apply -> FUN -> <Anonymous> -> <Anonymous> -> .local
Execution halted

Fitting and loading are both happening on the same system (university HPC cluster). I have no problems loading the stanfit object and then running other functions on this system (running extract(fit) for example) or loading the stanfit object on my local system (Mac OS 11.2.1). This also works fine if the model is fit and then loo is run within the same session (i.e. not from a .rds file).

HPC Cluster specs:

cat /etc/*-release

Cluster Manager v9.1
slave
NAME="Red Hat Enterprise Linux"
VERSION="8.2 (Ootpa)"
ID="rhel"
ID_LIKE="fedora"
VERSION_ID="8.2"
PLATFORM_ID="platform:el8"
PRETTY_NAME="Red Hat Enterprise Linux 8.2 (Ootpa)"
ANSI_COLOR="0;31"
CPE_NAME="cpe:/o:redhat:enterprise_linux:8.2:GA"
HOME_URL="https://www.redhat.com/"
BUG_REPORT_URL="https://bugzilla.redhat.com/"

REDHAT_BUGZILLA_PRODUCT="Red Hat Enterprise Linux 8"
REDHAT_BUGZILLA_PRODUCT_VERSION=8.2
REDHAT_SUPPORT_PRODUCT="Red Hat Enterprise Linux"
REDHAT_SUPPORT_PRODUCT_VERSION="8.2"
Red Hat Enterprise Linux release 8.2 (Ootpa)
Red Hat Enterprise Linux release 8.2 (Ootpa)

R environment specs:

R version 4.1.0 (2021-05-18) -- "Camp Pontanezen"
rstan (version 2.21.2)

Thanks for your help and developing such an awesome package! If this is an rstan and not a loo problem I'm happy to move the issue there.

Start with the loo(fit, moment_match = TRUE) entry point and the rstan/R/loo.R implementation linked in the report. Reproduce the failure by saving and reloading a stanfit before calling loo, then compare it with a same-session call. Done means the reloaded model completes moment matching without the invalid-model error, with a regression test covering the saved-and-loaded case.