Limitation: Xshell highlights whole lines, but you can simulate word-only highlighting by using lookarounds that force the match to be the primary content:
If you want, I can convert these conceptual rules into a ready-to-import Xshell highlight set file or give regex examples tuned to a specific log format. xshell highlight sets
Match exact phrases with spaces. Connection to .* timed out Limitation: Xshell highlights whole lines, but you can