So what changed? Go through your logs and see if there was something that changed at the time this error started occurring. But you will need to detect the cause quickly. However, check your moodledata folder, make sure that has not been tampered with. There should be some kind of log to detect if and when the moodledata folder was last accessed.
Be aware, this kind of error, can become a piece of string, that is, it just keeps getting longer and it is easy to be monstered by. Provisional response, reinstall the codebase. It is better to find the cause, but don't spend too much time on it. If nothing presents itself soon, refresh the codebase. If the error persists, then it is likely to be a database or moodledata folder error, which makes it easier to track.
Be aware, this kind of error, can become a piece of string, that is, it just keeps getting longer and it is easy to be monstered by. Provisional response, reinstall the codebase. It is better to find the cause, but don't spend too much time on it. If nothing presents itself soon, refresh the codebase. If the error persists, then it is likely to be a database or moodledata folder error, which makes it easier to track.