"Cannot read property 'upgrade' of undefined" when starting vue application - vue.js

Question

I have a project that's been running perfectly for a few months now, able to run npm run serve with no problem. I have to terminate and run the command again whenever I switch networks, but that has never been a problem and the server could always start again.
Until now - no matter what I do I can't get the server to start. I get this error:
npm run serve
> xbs#0.6.2 serve D:\workspace\[PROJECT]
> vue-cli-service serve
INFO Starting development server...
10% building 1/1 modules 0 active ERROR TypeError: Cannot read property 'upgrade' of undefined
TypeError: Cannot read property 'upgrade' of undefined
at Server.<anonymous> (D:\workspace\[PROJECT]\node_modules\webpack-dev-server\lib\Server.js:667:47)
at Array.forEach (<anonymous>)
at new Server (D:\workspace\[PROJECT]\node_modules\webpack-dev-server\lib\Server.js:666:22)
at serve (D:\workspace\[PROJECT]\node_modules\#vue\cli-service\lib\commands\serve.js:137:20)
at <anonymous>
at process._tickCallback (internal/process/next_tick.js:188:7)
at Function.Module.runMain (module.js:695:11)
at startup (bootstrap_node.js:191:16)
at bootstrap_node.js:612:3
The last time I got this message was when I forgot to set the .env files, but this is a previously working copy with all necessary files set, and no changes since the last working run.
I've even tried pulling a fresh copy of the repo, running npm i and setting all env files but it still fails.
npm run lint and npm run build can still work, only npm run serve.
Am I missing anything?

Answer 1

The problem was again with the .env files. My vue.config.js was trying to access a previously set environment variable that had since been removed.
Only strange thing was that there weren't any problems up til now?

Answer 2

I guess your devServer.proxy.target is empty!
You could set this configuration

Answer 3

For me it was that I forgot to create (or copy from another PC) the .env file after clone and try to run my project on another PC.

Answer 4

I also had this problem.
I used in the project process to get the backend address(for example):
devServer: {
proxy: {
'/api/back/*': {
target: process.env.VUE_APP_BACKEND_URL,
pathRewrite: {
'/api/back': '/api/back',
},
},
},
},
But there was no .evn file.
Just create .env file:
VUE_APP_BACKEND_URL= 0.0.0.0:2222 #example
Didn't notice the answer #Sergey.

Answer 5

I declared this environment variable in .env.dev, but the local build failed, and the same project ran without problems, so I created a new .env file and declared it in it
proxy: {
'/api': {
target: process.env.VUE_APP_PROXY,
changeOrigin: true,
pathRewrite: {
'^/api': ''
}
}
}