Spotlight fails to index on macOS High Sierra

Spotlight fails to index my internal SSD and I've failed up to now to find a way to fix it.

Note: it seems it happened after connecting an external spinning drive formatted in APFS (encrypted) but it might not be relevant. I recall reading something related to this problem, but can't find it anymore.

I've tried to force a reindex, both from the Spotlight System Preferences (adding the drive to Private and removing it) and from the command line without success.

The following errors are continually logged both for `mds` and `backupd` :

LaunchServices: store or url (null) was nil -10813, retrying

LaunchServices: Database mapping failed with result -10813, retries = 6

The retries loop between [0-9]

• This problem occurs for all existing users, incl. a newly created one;
• I reinstalled macOS High Sierra over the existing installation, but that didn't fix it;
• I ran DiskUtility to check for permission issues, and found none;
• I ran an utility called `EtreCheck` which comes up in different threads, to no avail. Nothing to clean up.

I'm a bit at a loss as I rely heavily on Spotlight in my daily workflows, both in the Finder and in Mail.

My last resort will be to reformat and clean install, but I'd rather avoid that if I can.