I was able to reproduce on our cloud instance (and some other instance like yours).

Atlassian Confluence v5.8.17:
https://confluence.atlassian.com/rest/api/content/search?cql=(space=%22TEAMCAL%22%20AND%20type=%22attachment%22)&limit=50&start=650&expand=container
Error: The entity spacedesc: Team Calendars for Confluence v.9 (240912700) is not ContentConvertible or API available.

https://confluence.atlassian.com/rest/api/content/search?cql=(space=%22SMT%22%20AND%20type=%22attachment%22)&limit=50&start=0&expand=container
Error: The entity spacedesc: Summit 2011 Speaker Guidelines v.7 (173768813) is not ContentConvertible or API available.

Public Apache Wiki v.5.8.4:
https://cwiki.apache.org/confluence/rest/api/content/search?cql=(space=%22ACE%22%20AND%20type=%22attachment%22)&limit=50&start=0&expand=container
Error: The entity spacedesc: Apache ACE v.2 (115845) is not ContentConvertible or API available.

On our Cloud instance v6.0.0 (coveord.atlassian.net):
https://coveord.atlassian.net/wiki/rest/api/content/search?cql=(space=%22SC%22%20AND%20type=%22attachment%22)&limit=50&start=0&expand=container
Error: The entity spacedesc: Sitecore v.1 (5865532) is not ContentConvertible or API available.

We have two instance on-premise 5.7.1 and 5.8.5 and they don't seem to be affected.