Why am I getting "Exit code 100 returned from process: file name 'tf'" - azure-devops

Question

I'm using Azure DevOps (visualstudio.com) and trying to get a Linux Build Agent up and running on a local server. This was working previously, but I upgraded the Linux server from Ubuntu 16.04 to 18.04. That broke my previous build agent.
So I removed the previous build agent and installed an up to date one. The new one registered easily, but when I try and run a build, it fails to check out sources.
Anybody know what might cause this problem, or where I can go for help? I don't manage our Azure DevOps. I'm just trying to get the Linux part working as it was.
Edit: I should note that running ./externals/tee/tf produces zero output (and returns with the same 100 code). Maybe it has something to do with java. I'm running Java(TM) SE Runtime Environment (build 12.0.2+10).
##[debug]Evaluating condition for step: 'Checkout'
##[debug]Evaluating: SucceededNode()
##[debug]Evaluating SucceededNode:
##[debug]=> True
##[debug]Result: True
##[section]Starting: Checkout
==============================================================================
Task : Get sources
Description : Get sources from a repository. Supports Git, TfsVC, and SVN repositories.
Version : 1.0.0
Author : Microsoft
Help : [More Information](https://go.microsoft.com/fwlink/?LinkId=798199)
==============================================================================
##[debug]Processed: ##vso[plugininternal.updaterepositorypath alias=__designer_repo;]/home/***/myagent/_work/1/s
##[debug]Repository requires to be placed at '/home/***/myagent/_work/1/s', current location is '/home/***/myagent/_work/1/s'
Prepending PATH environment variable with directory containing 'tf'.
##[debug]Processed: ##vso[task.prependpath]/home/***/myagent/externals/tee
##[debug]PATH: '/home/***/myagent/externals/tee:/home/***/bin:/home/***/.local/bin:/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin:/usr/games:/usr/local/games:/snap/bin'
Querying workspace information.
##[debug]tf workspaces -format:xml "-collection:https://mydomain.visualstudio.com/" -jwt:*** -noprompt
##[debug]Starting process:
##[debug] File name: 'tf'
##[debug] Arguments: 'workspaces -format:xml "-collection:https://mydomain.visualstudio.com/" -jwt:*** -noprompt'
##[debug] Working directory: '/home/***/myagent/_work/1/s'
##[debug] Require exit code zero: 'True'
##[debug] Encoding web name: ; code page: ''
##[debug] Force kill process on cancellation: 'False'
##[debug] Redirected STDIN: 'False'
##[debug] Persist current code page: 'False'
##[debug] Keep redirected STDIN open: 'False'
##[debug] High priority process: 'False'
##[debug]Updated oom_score_adj to 500 for PID: 1921.
##[debug]Process started with process id 1921, waiting for process exit.
##[debug]STDOUT/STDERR stream read finished.
##[debug]STDOUT/STDERR stream read finished.
##[debug]Finished process 1921 with exit code 100, and elapsed time 00:00:03.3458546.
##[error]Exit code 100 returned from process: file name 'tf', arguments 'workspaces -format:xml "-collection:https://mydomain.visualstudio.com/" -jwt:*** -noprompt'.
##[debug]Processed: ##vso[task.logissue type=error;]Exit code 100 returned from process: file name 'tf', arguments 'workspaces -format:xml "-collection:https://mydomain.visualstudio.com/" -jwt:*** -noprompt'.
##[debug]Processed: ##vso[task.complete result=Failed;]
##[debug] at Agent.Plugins.Repository.TeeCliManager.WorkspacesAsync(Boolean matchWorkspaceNameOnAnyComputer)
at Agent.Plugins.Repository.TfsVCSourceProvider.GetSourceAsync(AgentTaskPluginExecutionContext executionContext, RepositoryResource repository, CancellationToken cancellationToken)
at Agent.Plugins.Repository.CheckoutTask.RunAsync(AgentTaskPluginExecutionContext executionContext, CancellationToken token)
at Agent.PluginHost.Program.Main(String[] args)
##[section]Finishing: Checkout

Answer 1

When I saw that the tf command produced no output, I was pretty sure it had to be a java problem. I had tried many different versions of java and none of them worked. But with Oracle java 1.8.0_221 it finally worked.
I downloaded the .tar.gz file, extracted it, and overwrote the /usr/bin/java* symlinks to point to the 1.8 copies and then it worked.