Conradie, Francois F,
To ingest custom text logs from a linux or windows machines to sentinel workspace, you can refer this MS Doc for step-to-step detailed process.
So, coming to your issue there are few prerequisites for ingestion before connecting it through AMA. You need to have a specific RBAC permissions like "Monitoring contributor" to avoid any role related conflicts while log ingestion.
Refer MS Doc for more related information.
Now,
Does AMA need specific file permissions on the folder it is picking the log files up from?
As I given above, you can check access control with "getfacl" command and verify if the reader access existed for the specific log files/directories.
Also, make sure that the DCR has been correctly configured to read from the required file path and also check that the AMA status is successfully running to collect and send the logs.
Hope this helps.
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