Test without a plugin — only for you
From any error row, disable a suspect plugin for your own signed session only. Visitors keep running the site exactly as normal. Reproduce the problem; if it stops for you while other visitors still hit it, that is strong evidence — the verdict is automatic.
Whole-site tests
Bisect halves the plugin list per answer to corner a conflict in a few rounds. Safe mode answers “does the site work at all without third-party plugins?”. The default-theme test answers “is it the theme?”. All of it in your session only, in memory, never touching the database.
Confirmed ≠ resolved
Isolation confirms the culprit; it does not fix it. The plugin keeps affecting visitors until you update or deactivate it for real — then mark it resolved.