Bonjour @Mageunic et merci de ta réponse !
Voilà quelques infos sur mon serveur :
VPS de chez OVh
nginx version: nginx/1.18.0 (Ubuntu)
Distributor ID: Ubuntu
Description: Ubuntu 20.04.2 LTS
Release: 20.04
Nextcloud version 20.0.7
En ce qui concerne la commande évoquée plus haut, je n’arrivais pas à l’utiliser avec www-data, j’ai donc lancé : sudo -u nextcloud php occ db:add-missing-indices. C’est là que tout a planté 
Idem pour celle que tu m’as indiqué, je te poste le résultat :
sudo -u nextcloud php occ maintenance:repair
- Repair MySQL collation
- All tables already have the correct collation -> nothing to do
- Repair mime types
- Clean tags and favorites
- 0 tags of deleted users have been removed.
- 0 tags for delete files have been removed.
- 0 tag entries for deleted tags have been removed.
- 0 tags with no entries have been removed.
- Repair invalid shares
- Move .step file of updater to backup location
- Add move avatar background job
- Repair step already executed
- Add preview cleanup background jobs
- Fix potential broken mount points
- Repair language codes
- Install new core bundle components
- Add log rotate job
- Clear frontend caches
- Image cache cleared
- SCSS cache cleared
- JS cache cleared
- Clear every generated avatar on major updates
- Add preview background cleanup job
- Queue a one-time job to cleanup old backups of the updater
- Cleanup invalid photocache files for carddav
- Add background job to cleanup login flow v2 tokens
- Remove potentially over exposing share links
- No need to remove link shares.
- Clear access cache of projects
- Reset generated avatar flag
- Keep legacy encryption enabled
- Check encryption key format
- Remove old dashboard app config data
- Update name of the stored view
- Fix component of birthday calendars
- 5 birthday calendars updated.
- Regenerating birthday calendars to use new icons and fix old birthday events without year
- Repair step already executed
- Fix broken values of calendar objects
0 [->--------------------------]
- Registering building of calendar search index as background job
- Repair step already executed
- Registering background jobs to update cache for webcal calendars
- Added 0 background jobs to update webcal calendars
- Registering building of calendar reminder index as background job
- Repair step already executed
- Clean up orphan event and contact data
- 0 events without a calendar have been cleaned up
- 0 properties without an events have been cleaned up
- 0 changes without a calendar have been cleaned up
- 0 cached events without a calendar subscription have been cleaned up
- 0 changes without a calendar subscription have been cleaned up
- 0 contacts without an addressbook have been cleaned up
- 0 properties without a contact have been cleaned up
- 0 changes without an addressbook have been cleaned up
- Remove activity entries of private events
- Removed 0 activity entries
- Fix the share type of guest shares when migrating from ownCloud
- Copy the share password into the dedicated column
- Set existing shares as accepted
- Purify and migrate collected mail addresses
0 [----->----------------------]
- Insert background jobs for all accounts
0 [--------->------------------]
- Make Mail itinerary extractor executable
- Migrate Mail provisioning config from config.php to the database
- Create or update provisioned Mail accounts
- No Mail provisioning config set
- Update OAuth token expiration times
- Create help command
- Invalidate access cache for projects conversation provider
- Invalidation not required
- Switches from default updater server to the customer one if a valid subscription is available
- Repair step already executed
- Send an admin notification if monthly report is disabled
- Add background job to check for backup codes
- Populating added database structures for workflows
Cela n’a apporté aucun changement.