Many thanks to David for reporting this. You may have seen the error:
Call to undefined function idna_maps_not_std3()
When attempting to view or start a thread. This was due to a bug (sort of) introduced by SMF 2.1.6.
idna_maps_not_std3() is a function related to the processing and validation of domain names that contain non-ASCII characters. It looks like SMF 2.1.6 has a stricter dependency on this than previous versions.
Critically, the function depends on a package that provides the internationalisation extension for PHP, php7.4-intl in this case. This has been installed now and the problem has been resolved.
A lot more info here (https://www.simplemachines.org/community/index.php?topic=592100.msg4192458#msg4192458) if interested, in a post from one of the lead developers on the SMF forum.