That got me one more step. After that uglifier-js was missing. Installed that. Now this is the error and the admin page is completely gone...
Error: Failed to lookup view "500" in views directory "/opt/mangolassi2/public/templates"
at EventEmitter.render (/opt/mangolassi2/node_modules/express/lib/application.js:579:17)
at ServerResponse.render (/opt/mangolassi2/node_modules/express/lib/response.js:961:7)
at ServerResponse.res.render (/opt/mangolassi2/src/middleware/middleware.js:337:10)
at /opt/mangolassi2/src/routes/index.js:223:9
at /opt/mangolassi2/src/middleware/middleware.js:192:12
at /opt/mangolassi2/node_modules/async/lib/async.js:589:17
at done (/opt/mangolassi2/node_modules/async/lib/async.js:126:15)
at /opt/mangolassi2/node_modules/async/lib/async.js:32:16
at /opt/mangolassi2/node_modules/async/lib/async.js:586:21
at EventEmitter.render (/opt/mangolassi2/node_modules/express/lib/application.js:581:14)
Which is the error that you got too. It appears that you never solved it and just did a fresh install instead?
@scottalanmiller Yes, my installation is just a test installation so it didn't matter even if I lose anything so, I did a fresh install.
That seems to be the trend. The upgrade process just doesn't work. Even with tons of manual intervention every time, it's a new adventure each time.
I did the updates on another forum that we run and they went smoothly.
@Kalbaskin said:
I did:
npm i nodebb-theme-persona
npm i nodebb-theme-lavender
npm i uglify-jsIt helped me.
Didn't work, same error. 
Not sure what happened but tons of files were missing. I did a full upgrade cycle again starting from scratch and all was fine. No idea why there was an issue but all is fixed now. Normal upgrade worked.
@scottalanmiller try npm i express
oh fixed. np
