I was receiving various vite proxy & connection-related errors e.g.
│ frontend │ 9:10:42 PM [vite] http proxy error: │ frontend │ Error: socket hang up │ frontend │ at connResetException (node:internal/errors:717:14) │ frontend │ at Socket.socketOnEnd (node:_http_client:519:23) │ frontend │ at Socket.emit (node:events:523:35) │ frontend │ at endReadableNT (node:internal/streams/readable:1367:12) │ frontend │ at process.processTicksAndRejections (node:internal/process/task_queues:82:21)

but the initial error was:
│ backend │ [shopify-app/ERROR] Could not check if session was valid: TypeError: scopesArray.map is not a function | {shop: textline-integration-test-store.myshopify.com}

& I finally just solved it. I just needed to upgrade my version of shopify-app-express in web/package.json to 2.0.0. I’ll try to let the github community know about it. Credit Shopify/shopify-app-template-node#1208 (comment)

It took me a while to find this answer, so I just figured that I'd post this under "issues".