Hi Michailidis, Tony (DFO/MPO),
Welcome to the Microsoft Q&A Platform! Thank you for asking your question here.
The possible causes for the node to go from a Ready condition to ReadonlyFilesystem condition are if the disk size is full, it may change to read only mode, if there are any node's underlying corruption or hardware issues.
Please try below troubleshooting steps which might help you:
Check the Node status and describe the Node which is in NotReady state. Once try restarting the Node. Try unmounting and remounting the file system. Check the disk size if it is full df -h
For more information, please check: Node Problem Detector (NPD) in Azure Kubernetes Service (AKS) nodes
Troubleshoot Linux virtual machine boot issues due to filesystem errors
If you have further queries, please do let us know.