I'm using dovecot 2.3.13,  solr 8.x. and roundcube 1.5.1. However when I do a search I get the the Error: fts_solr: received invalid uid, search results are ok. 

Thanks

Presumably one of the uid returned from solr could not be decoded (i.e. converted to an int). The offending uid should have been printed as part of the message. If you need to get more info you could investigate what is being returned by solr running the exact same query from a browser if you're able to retrieve it from the solr log.

Alternatively you could set rawlog_dir in the fts_solr dovecot settings and then look at what is being sent back from solr in the .in file that is logged.

https://doc.dovecot.org/settings/plugin/fts-solr-plugin/?highlight=fts%20dovecot%20plugin

John