SOLCLIENT_SUBCODE_DATA_OTHER - Exceeded Spool File Limit

"{'caller_description': 'From service event callback', 'return_code': 'Unknown (503)', 'sub_code': SOLCLIENT_SUBCODE_DATA_OTHER', 'error_info_sub_code': 28, 'error_info_contents': "Exceeded Spool File Limit - Topic 'my/topic'"

Still haven't resolved this issue as the docs are quite ambiguous about this error code. Solutions I've tried include:

1. Increase Event-broker wide Guaranteed Message Maximum Message Spool Usage (MB)
2. Increase Message VPN and queue Maximum Message Spool Usage (MB)

Help please!