Upgrade GitLab CE 9.4 -> 9.5 fails on step "Update node dependencies and recompile assets"

Upgrade GitLab CE 9.4 -> 9.5 fails on step "Update node dependencies and recompile assets" when running

bundle exec rake yarn:install gitlab:assets:clean gitlab:assets:compile RAILS_ENV=production NODE_ENV=production

with error:

yarn install v0.21.3
[1/4] Resolving packages...
[2/4] Fetching packages...
warning fsevents@1.0.17: The platform "linux" is incompatible with this module.
info "fsevents@1.0.17" is an optional dependency and failed compatibility check. Excluding it from installation.
[3/4] Linking dependencies...
[4/4] Building fresh packages...
Done in 32.60s.
warning fsevents@1.0.17: The platform "linux" is incompatible with this module.
error "webpack#ajv" is wrong version: expected "5.2.0", got "4.11.2"
error Found 1 errors.
Error: You have unmet dependencies. (`yarn check` command failed)
Run `yarn install` to install missing modules.

Following this guide: https://github.com/gitlabhq/gitlabhq/blob/master/doc/update/9.4-to-9.5.md

Please help, quite urgent since our GitLab instance is now down and unsable.

Thanks in advance