cc @Esanders @Tchanders, maybe related to work in T405080: Convert LQT pages on enwiktionary to Flow?

I don't see how this could be Flow's fault; all Flow does is use a standard (albeit deprecated) core UI to make an edit.

The errors were at least triggered by our maintenance script runs.

Without being very familiar with Flow, and just from a cursory glance, there are a few places that get a timestamp via wfTimestamp (which returns false if the input is wrongly formed) that may not check the timestamp is a string before using it...

Usages: https://codesearch.wmcloud.org/search/?q=wfTimestamp%5C%28&files=&excludeFiles=&repos=Extension%3AFlow

Oh, right. Tracing the call stack,

		$parserOutput = $contentHandler->getParserOutput( $content, $cpoParams );
[...]
		if ( $parserOutput->getRevisionTimestamp() === null && $revTimestamp !== null ) {

(from https://github.com/wikimedia/mediawiki/blob/master/includes/content/ContentHandler.php)

			$this->fillParserOutput(
				$content,
				$cpoParams,
				$po
			);

From https://github.com/wikimedia/mediawiki-extensions-Flow/blob/master/includes/Content/BoardContentHandler.php

		if ( $revId === null ) {
			$wikiPage = MediaWikiServices::getInstance()->getWikiPageFactory()->newFromTitle( $title );
			$timestamp = $wikiPage->getTimestamp();
		} else {
			$timestamp = MediaWikiServices::getInstance()->getRevisionLookup()
				->getTimestampFromId( $revId );
		}
		$output->setTimestamp( $timestamp );

Where setTimestamp is a deprecated alias for setRevisionTimestamp

You can see from Line 1 of the call stack that $revId almost certainly isn't null (if it were then null would have been passed there rather than a revisionRecord), so we call getTimestampFromId

	 * @return string|false False if not found
	 */
	public function getTimestampFromId( $id, $flags = 0 ) {

That's probably where the false is coming from, and that seems to be reading from a replica database. So looks like another database replication issue. Sigh.