Crawl failing due to Accessibility auditor

Hi,

We have a number of collections which use the accessibility auditor, and all of these collections have started failing on crawl with this error in the update.log -

2021-02-08 10:31:10,172 [main] WARN accessibility.RecordAccessibilityAuditorHistory - Error while generating accessibility auditor historical data for _default
com.funnelback.adminapi.client.ApiException: Server Error

We are running Funnelback 15.12 self-hosted. The crawls complete successfully when the accessibility auditor is switched off.

Has anyone seen this before?

Thanks,
Ralph