Agent has stopped running executables

A previously functional agent has stopped being able to launch executables as part of build steps.

I created a minimal reproduce case config that consists of a single "Command Line | Executable With Parameters" step:
"C:\Program Files\Mozilla Firefox\firefox.exe"

On our other still-functioning agent (which is also the server), this config launches Firefox before completing. The problematic agent produces identical log output:

but does not launch Firefox. The same command run from the terminal works as expected.

The only log file in Teamcity/buildAgent/logs that shows modifications after the config runs is wrapper.log, which has the following entries:

INFO | jvm 1 | 2021/10/12 17:54:39 | [2021-10-12 17:54:39,284] WARN - dDirectoryBasedCleanupRegistry - Failed to remove file C:\TeamCity\buildAgent\temp\.old\buildTmp_0\03dfb156-5d7d-447d-8d80-8f9e2f5a3a6d.tmp.node
INFO | jvm 1 | 2021/10/12 17:54:39 | [2021-10-12 17:54:39,293] WARN - dDirectoryBasedCleanupRegistry - Failed to remove file C:\TeamCity\buildAgent\temp\.old\buildTmp_0\05d2acbc-3a27-44e8-afcf-1f6c7a767e72.tmp.node
INFO | jvm 1 | 2021/10/12 17:54:39 | [2021-10-12 17:54:39,302] WARN - dDirectoryBasedCleanupRegistry - Failed to remove file C:\TeamCity\buildAgent\temp\.old\buildTmp_0\0f042dbd-755b-4e13-8d35-59d33fb27e43.tmp.node
INFO | jvm 1 | 2021/10/12 17:54:39 | [2021-10-12 17:54:39,311] WARN - dDirectoryBasedCleanupRegistry - Failed to remove file C:\TeamCity\buildAgent\temp\.old\buildTmp_0\29921510-88b4-4beb-8a54-72d5abc7b096.tmp.node
INFO | jvm 1 | 2021/10/12 17:54:39 | [2021-10-12 17:54:39,320] WARN - dDirectoryBasedCleanupRegistry - Failed to remove file C:\TeamCity\buildAgent\temp\.old\buildTmp_0\2a1b988d-1488-4efc-9899-c5a42e95f2c3.tmp.node
INFO | jvm 1 | 2021/10/12 17:54:39 | [2021-10-12 17:54:39,329] WARN - dDirectoryBasedCleanupRegistry - Failed to remove file C:\TeamCity\buildAgent\temp\.old\buildTmp_0\4b22811d-4772-4df8-aa2d-c0e80ff431bd.tmp.node
INFO | jvm 1 | 2021/10/12 17:54:39 | [2021-10-12 17:54:39,338] WARN - dDirectoryBasedCleanupRegistry - Failed to remove file C:\TeamCity\buildAgent\temp\.old\buildTmp_0\7d4a06ba-1721-4fae-b00a-84b7befbcc79.tmp.node
INFO | jvm 1 | 2021/10/12 17:54:39 | [2021-10-12 17:54:39,338] WARN - dDirectoryBasedCleanupRegistry - Failed to remove file C:\TeamCity\buildAgent\temp\.old\buildTmp_0

So far I've tried:
- restarting the agent and the TeamCity server
- rolling back windows to a system restore point from a time when the agent was still functional.

We are running TeamCity 2020.2.2, and will try upgrading to the most recent version next.

Any help in diagnosing this would be greatly appreciated.