Lewatle,
Did you see this error?
12-Oct-2021 12:54:42.464 SEVERE [main] org.apache.catalina.core.StandardServer.await Failed to create server shutdown socket on address [localhost] and port [8005] (base port [8005] and offset [0])
java.net.BindException: Address already in use (Bind failed)
I'm not a tomcat/java expert, but that port, 8005, should not already be in use when Tomcat starts. Is tomcat running twice? Is something else using that port? That could be a clue to what's going on. It could also mean that's why SOLR is having trouble.
Second, this is a big clue that something is wrong with SOLR:
2021-10-13 01:15:01,761 ERROR unknown unknown org.dspace.app.sitemap.GenerateSitemaps @ Failed getting entitytype through solr for item d137c57a-caff-4dde-a401-5fd658584dc0: Error from server at http://localhost:8983/solr/search: Expected mime type application/octet-stream but got text/html. <html>
What happens when you try to go to http://localhost:8983/solr/ or http://localhost:8983/ ? You should see the SOLR manager page at the first URL. Tim is right in that the DSpace front end is not able to communicate to SOLR. I also had the same sort of "Error Fetching Sub-Communities" and I resolved it in a way that was appropriate for my situation, but it was related to SOLR.
--Joel