[TYPO3-Solr] Upgrade 6.2 -> 7.6 problem

[Dennis Lümkemann]

Hello all,

after upgrading my Typo3 installation from 6.2 to 7.6, the Extension Manager stays blank due to
PHP Fatal error:  Class 'ApacheSolrForTypo3\\Solrfal\\Migrations\\RenameAdditionalFields' not found in /www/test76/typo3temp/ExtensionManager/UpdateScripts/ext_update9fc5a2af695311dfaee336d59f2d134a.php on line 52

The file RenameAdditionalFields.php is there in typo3conf/ext/solrfal/Classes/Migrations.

Why could this be?

solrfal_2.1.3_201604211118
Typo3 7.6.15

Thanks
Dennis