Additionally, here the errors I get in logs:

Dovecot:

Dec 09 09:21:09 imap(jom@grosjo.net)<3349><DiRnXpN8Lux/AAAB>: Error: fts_solr: received invalid uid '0'
Dec 09 09:21:10 imap(jom@grosjo.net)<3349><DiRnXpN8Lux/AAAB>: Error: fts_solr: received invalid uid '0'

or

11 03:36:03 indexer-worker(jom@grosjo.net)<2093><icWMJaIwD1znEgAA0thIag:GPBOCKMwD1wtCAAA0thIag>: Error: fts_solr: Indexing failed: 500 Server Error

This looks like a permissions issue.  Are you using NFS?