Tabbing through the page helped me find the issue as it highlighted 'POP Retrieval Download Path'.
This was set to;
S:\PopRetrieval
Changing it to the following fix the issue
S:\PopRetrieval\
This has been set to this for at least 3 years. Maybe a better way to highlight such errors? As tabbing through each setting to find the error is not something that would be obvious to do.