SyntaxError: Failed to execute 'json' on 'Response': Unexpected token '<', "<!DOCTYPE "... is not valid JSON

[ops-handler]

Error

Failed to execute 'json' on 'Response': Unexpected token 'R', "Retry later
" is not valid JSON

Stack trace

../app/javascript/components/ErrorBoundary.tsx:108 in d
../node_modules/react-dom/cjs/react-dom.production.min.js:243 in Qj
../node_modules/react-dom/cjs/react-dom.production.min.js:285 in Hk
../node_modules/react-dom/cjs/react-dom.production.min.js:281 in d
../node_modules/scheduler/cjs/scheduler.production.min.js:13 in J
../node_modules/scheduler/cjs/scheduler.production.min.js:14 in R

Tracking Details

• Project: exercism-js
• Sentry: SyntaxError: Failed to execute 'json' on 'Response': Unexpected token '<', "<!DOCTYPE "... is not valid JSON
• Culprit: d(app/javascript/components/ErrorBoundary)
• First seen: Feb 9, 2026