From my experience, DirBuster tends to be used more often by a real human attacker instead of a bot scanning everything it could find. 100% automated scanning typically only looks for a handful of directory names to exploit specific vulnerabilities, while DirBuster is usually used to try and bruteforce from a large dictionary.
So, this could indicate that an attacker has specifically singled out your site. The attacker may or may not be dedicated, and they may or may not be particularly intelligent.
I am unsure if you're saying that there are a lot of logs like that, or if you only noticed the one request. If there's only one request, then it's just looking for that one directory, in which case it's almost certainly a completely automated attack. If you see more than a few requests with that user-agent, continue reading.
What you should first do is research the IP address and determine if it's a proxy of some kind (HTTP proxy, VPN, Tor exit node), or if it seems to be a residential IP. Then,
grep your access logs for that IP for the past week or so and see if it's made any prior visits, or visits after this scan. If it's a fairly generic proxy, past visits from that IP may not be from the same entity, so keep that in mind.
And most importantly of all, do a quick audit of everything in your webroot directory (typically
/var/www/html on Linux distributions, no idea for IIS). Disable directory indexing, and look for anything sensitive in there you may have forgotten about, like a backup
.sql file containing source code or a database dump.
If the only things accessible to the public are the actual applications you want users to use, then there is really nothing to be concerned with here. Other than that, the only remaining concern is if the attacker appears to be a real human, and if so, if they performed any other actions in an attempt to breach your server.