Unable to import to Omeka S via Omeka 2 importer

I’m trying to import the items and collections from an Omeka 2 site into Omeka S. Each time I get the following output in the log file:

2021-11-24T20:22:08+00:00 ERR (3): HTTP problem: 403 Forbidden
Fatal error: Uncaught Error: Call to a member function toString() on bool in /home/dslplatt/collections.dsl-plattsburgh.org/modules/Omeka2Importer/src/Job/Import.php:453
Stack trace:
#0 /home/dslplatt/collections.dsl-plattsburgh.org/modules/Omeka2Importer/src/Job/Import.php(162): Omeka2Importer\Job\Import->hasNextPage(Object(Laminas\Http\Response))
#1 /home/dslplatt/collections.dsl-plattsburgh.org/modules/Omeka2Importer/src/Job/Import.php(91): Omeka2Importer\Job\Import->importCollections(Array)
#2 /home/dslplatt/collections.dsl-plattsburgh.org/application/src/Job/DispatchStrategy/Synchronous.php(34): Omeka2Importer\Job\Import->perform()
#3 /home/dslplatt/collections.dsl-plattsburgh.org/application/src/Job/Dispatcher.php(105): Omeka\Job\DispatchStrategy\Synchronous->send(Object(Omeka\Entity\Job))
#4 /home/dslplatt/collections.dsl-plattsburgh.org/application/data/scripts/perform-job.php(66): Omeka\Job\Dispatcher->send(Object(Omeka\Entity\Job), Object(Omeka\Job\DispatchStrategy\Synchronous))
#5 {main}
thrown
in /home/dslplatt/collections.dsl-plattsburgh.org/modules/Omeka2Importer/src/Job/Import.php on line 453

The source Omeka is at https://plattsburghrocks.org; the destination is at https://collections.dsl-plattsburgh.org.

Any help would be much appreciated!