dovecot fts-solr + solr 8.7.0 upgrade: "Indexing failed: 401 Unauthorized" + "Transaction commit failed: FTS transaction commit failed: backend deinit" ?

[PGNet Dev]

I've built a new dovecot + fts-solr instance; I've now picked up & am running the recently released solr 8.7.0.

In a test account, I've one message -- in the 'Drafts' folder.

On exec of fts 'index'

	doveadm index -u testuser at example.com -q '*'

I get a

	401 unauthorized

indexing failure, only for the one dir+message in 'Drafts',

	2020-11-13 05:43:21 indexer-worker(testuser at example.com)<8MKVNHmNrl/DJgEA+IOfAw>: Info: Indexed 0 messages in Queue
	2020-11-13 05:43:21 indexer-worker(testuser at example.com)<OHfzNHmNrl/DJgEA+IOfAw>: Info: Indexed 0 messages in testing
	2020-11-13 05:43:21 indexer-worker(testuser at example.com)<yMxJNXmNrl/DJgEA+IOfAw>: Info: Indexed 0 messages in Junk
	2020-11-13 05:43:21 indexer-worker(testuser at example.com)<uDCbNXmNrl/DJgEA+IOfAw>: Info: Indexed 0 messages in Trash
	2020-11-13 05:43:21 indexer-worker(testuser at example.com)<kA/uNXmNrl/DJgEA+IOfAw>: Info: Indexed 0 messages in Sent
	2020-11-13 05:43:22 indexer-worker(testuser at example.com)<cEw/NnmNrl/DJgEA+IOfAw>: Error: fts_solr: Indexing failed: 401 Unauthorized
	2020-11-13 05:43:22 indexer-worker(testuser at example.com)<cEw/NnmNrl/DJgEA+IOfAw>: Error: fts_solr: Indexing failed: 401 Unauthorized
	2020-11-13 05:43:22 indexer-worker(testuser at example.com)<cEw/NnmNrl/DJgEA+IOfAw>: Error: Mailbox Drafts: Transaction commit failed: FTS transaction commit failed: backend deinit (attempted to index 1 messages (UIDs 3..3))
	2020-11-13 05:43:22 indexer-worker(testuser at example.com)<2OUMCHqNrl/DJgEA+IOfAw>: Info: Indexed 0 messages in INBOX

I get NO output atm in my solr logs @ this 'index'; just, atm, in the dovecot logs, above

I've checked all my usual perms.
Indexing _was_ working with a prior solr v8.6.3 ...

Either I've misconfigured some perm, and am missing it, or new-solr-version v8.7.0 is causing me some grief.

(1) Can anyone yet verify Doveoct/fts-solr working with solr 8.7.x?
(2) What logging config in Dovecot gets me more/detailed output for the fts_solr fail?
(3) Any obvious clues as to what, specifically, is the source of this prob?