What do I need to make collabora work with nextcloud?

I’m running Collabora and Nextcloud via docker compose through pfsense, let’s encrypt and HAproxy.

Collabora shows connected in Nextcloud–>Settings–>Collabora. When I browse to http://collabora.mycustomdomain.com I get “OK” in the upper left of the screen. Also, I can see Collabora running by issuing “netstat -tupln” as it is listening on TCP port 3306, so collabora is certainly running. However when I try to open or create, say, a spreadsheet file, I just get a blank nextcloud white screen. Below are many screenshots of what I thought to be relevant. What other information can I provide?

docker-compose.yml
Collabora settings showing connected
Nextcloud–>Settings–>Logging (also an expanded version of that below)

Exception: strtolower() expects parameter 1 to be string, null given

/var/www/html/lib/private/AppFramework/App.php - line 137:

OC\AppFramework\Http\Dispatcher->dispatch(OC\Core\Cont ... {}, "getPreviewByFileId")

/var/www/html/lib/private/AppFramework/Routing/RouteActionHandler.php - line 47:

OC\AppFramework\App::main("OC\\Core\\C ... r", "getPreviewByFileId", OC\AppFramew ... {}, { _route: "c ... "})

<<closure>>

OC\AppFramework\Routing\RouteActionHandler->__invoke({ _route: "c ... "})

/var/www/html/lib/private/Route/Router.php - line 297:

call_user_func(OC\AppFramew ... {}, { _route: "c ... "})

/var/www/html/lib/base.php - line 1007:

OC\Route\Router->match("/core/preview")

/var/www/html/index.php - line 37:

OC::handleRequest()

Caused by TypeError: strtolower() expects parameter 1 to be string, null given

/var/www/html/lib/private/Http/Client/Client.php - line 162:

strtolower(null)

/var/www/html/lib/private/Http/Client/Client.php - line 293:

OC\Http\Client\Client->preventLocalAddress("collabora.d ... g", { timeout: 1 ... ]})

/var/www/html/custom_apps/richdocuments/lib/Preview/Office.php - line 92:

OC\Http\Client\Client->post("collabora.d ... g", { timeout: 1 ... ]})

/var/www/html/lib/private/Preview/ProviderV1Adapter.php - line 54:

OCA\Richdocuments\Preview\Office->getThumbnail("Welcome to Nextcloud Hub.docx", 4096, 4096, false, OC\Files\View {})

/var/www/html/lib/private/Preview/GeneratorHelper.php - line 63:

OC\Preview\ProviderV1Adapter->getThumbnail(OC\Files\Node\File {}, 4096, 4096)

/var/www/html/lib/private/Preview/Generator.php - line 235:

OC\Preview\GeneratorHelper->getThumbnail(OC\Preview\ProviderV1Adapter {}, OC\Files\Node\File {}, 4096, 4096)

/var/www/html/lib/private/Preview/Generator.php - line 141:

OC\Preview\Generator->getMaxPreview(OC\Files\Sim ... {}, OC\Files\Node\File {}, "application ... t", "")

/var/www/html/lib/private/Preview/Generator.php - line 107:

OC\Preview\Generator->generatePreviews(OC\Files\Node\File {}, [ { width: 3 ... }], "application ... t")

/var/www/html/lib/private/PreviewManager.php - line 190:

OC\Preview\Generator->getPreview(OC\Files\Node\File {}, 32, 32, true, "fill", null)

/var/www/html/core/Controller/PreviewController.php - line 170:

OC\PreviewManager->getPreview(OC\Files\Node\File {}, 32, 32, true, "fill")

/var/www/html/core/Controller/PreviewController.php - line 143:

OC\Core\Controller\PreviewController->fetchPreview(OC\Files\Node\File {}, 32, 32, false, true, "fill")

/var/www/html/lib/private/AppFramework/Http/Dispatcher.php - line 170:

OC\Core\Controller\PreviewController->getPreviewByFileId(21, 32, 32, false, true, "fill")

/var/www/html/lib/private/AppFramework/Http/Dispatcher.php - line 100:

OC\AppFramework\Http\Dispatcher->executeController(OC\Core\Cont ... {}, "getPreviewByFileId")

/var/www/html/lib/private/AppFramework/App.php - line 137:

OC\AppFramework\Http\Dispatcher->dispatch(OC\Core\Cont ... {}, "getPreviewByFileId")

/var/www/html/lib/private/AppFramework/Routing/RouteActionHandler.php - line 47:

OC\AppFramework\App::main("OC\\Core\\C ... r", "getPreviewByFileId", OC\AppFramew ... {}, { _route: "c ... "})

<<closure>>

OC\AppFramework\Routing\RouteActionHandler->__invoke({ _route: "c ... "})

/var/www/html/lib/private/Route/Router.php - line 297:

call_user_func(OC\AppFramew ... {}, { _route: "c ... "})

/var/www/html/lib/base.php - line 1007:

OC\Route\Router->match("/core/preview")

/var/www/html/index.php - line 37:

OC::handleRequest()