Hi Vero,
Thank you for your response and suggestions!
My Mac is shutdown on a daily basis without a change in Alfred behavior (my clean install actually goes back a few weeks - I just didn't have the time to address the problem).
Regarding Dropbox, what you see is not the installer but my Dropbox volume which is mounted at startup with CloudMounter. My sync folder is indeed located on this volume. This is my only Mac so no conflict there - and the preferences certainly appear synced, my pre-clean install custom web searches all being there - images and all - and working fine via the test button (= I can access and edit them). It's really just the keywords that don't work anymore for some reason.
Furthermore, the default Alfred workflows' keywords are not recognized either, nor new workflows' specifically imported to investigate the matter.
On the other hand, default Alfred web searches work just fine.
Please let me know if there's anything I can test further!
Best,
Alexis