Hi,
No need to change index settings, R81.10 and R81.20 run on same SOLR version so no need to re-index.
Though if you can open log files but see nothing without it, it's 99% indexer issue.
1. If you did an upgrade and your log server is not your MGMT server, please make sure to perform "Install database" operation.
2. You can stop and restart indexer to see if it solves the issue. "stopIndexer ; startIndexer ;"
3. If 1 and 2 doesn't solve the issue, look for errors in $INDEXERDIR/log/log_indexer.elg.
Kind regards, Amir Senn