This problem appears to be connected to out of date and/or incorrect manifest.csv files that live under the datamodel_summaries directories in every index directory.
Remove those files with a find/exec rm. Splunk will rebuild this file in a few minutes, and it does not require a restart of the indexer or any cluster maint mode. The error message should then cease. If it doesn't, try removing them a second time.
It also led, in our case, to incomplete data models due to Splunk thinking that the datamodel volume sizes were, in some cases 10x their actual size.
Here's SPL to find this error. Note the two spaces between "WARN and CMslave"
index=_internal sourcetype=splunkd "WARN CMSlave" *datamodel_summary*
It seems that it can take multiple removals of the manifest.csv files to persuade some indexers to rewrite them correctly.
This rest call shows _splunk_summaries volume sizes:
| rest splunk_server=<something to identify your indexers> /services/data/index-volumes
| fields splunk_server, title, total_size, max_size, volume_path
|search title=_splunk_summaries
| eval total_size_gb = if(isnull(total_size), "-", round(total_size / 1024, 2))
| eval max_size_gb = if(isnull(max_size) OR max_size = "infinite", "unlimited", round(max_size / 1024, 2))
| eval disk_usage_gb = total_size_gb
| fields splunk_server, title, disk_usage_gb, max_size_gb, volume_path
| sort - disk_usage_gb
| rename title as Volume, disk_usage_gb as "Volume Usage (GB)", max_size_gb as "Volume Capacity (GB)", volume_path as "Volume Path"
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