After neglecting upgrades so long I upgraded to newest versions, to fix broken bootstrap links. All went better than last update from 1.1.2 to 1.5.x. First I did backups, and test upgrade, and then live version, all ok. Now about 12 hours later forum crashed.
I also upgraded Node to newest LTS version (8.11.1), previous version was running with 6.x.
At log I have 4 of these:
2018-04-07T11:10:49.148Z  - info: [app] Shutdown (SIGTERM/SIGINT) Initialised.
2018-04-07T11:10:49.147Z  - error: TypeError: Cannot read property 'hasOwnProperty' of null
at postParse (/nodebb/node_modules/nodebb-plugin-markdown/index.js:181:15)
at nextTask (/nodebb/node_modules/async/dist/async.js:5310:14)
at next (/nodebb/node_modules/async/dist/async.js:5317:9)
at _combinedTickCallback (internal/process/next_tick.js:138:11)
at process._tickCallback (internal/process/next_tick.js:180:9)
2018-04-07T11:10:49.160Z  - info: [app] Web server closed to connections.
2018-04-07T11:10:49.169Z  - info: [app] Database connection closed.
2018-04-07T11:10:49.170Z  - info: [app] Shutdown complete.
[cluster] Child Process (39) has exited (code: 1, signal: null)
[cluster] Spinning up another process...
3 restarts in 10 seconds, most likely an error on startup. Halting.