After upgrading to TeamCity 2017.2 my SVN checkout began failing on my build agent with this error:
svn: E200030: There are unfinished transactions detected in '[... folder ...]'
The agent is running on Windows Server 2012 R2 and TeamCity Professional 2017.2 (build 50574)
Initial Googling and browsing on Stack Overflow didn't find an answer yet.
After further Googling I found this bug on the TeamCity YouTrack:
https://youtrack.jetbrains.com/issue/TW-49325 specifically this comment https://youtrack.jetbrains.com/issue/TW-49325#comment=27-2472122
I had TortoiseSVN 1.9 installed on the build agent.
Disabling the Tortoise icon overlays for my build agent folders under Settings -> Icon Overlays -> Exclude Paths allowed my checkout to continue successfully again.
Looks like the problem is caused by a TortoiseSVN client when it monitors SVN folders,