...
https://<your_tenant_name>.sharepoint.com (API resources)
https://login.microsoftonline.com/ (API Authentication)
As a user I’m not able to create a folder hierarchy where the total path is longer than 400 characters
The entire decoded file path, including the file name, can't contain more than 400 characters for SharePoint. The limit applies to the combination of the folder path and file name after decoding. (https://learn.microsoft.com/en-us/office365/servicedescriptions/sharepoint-online-service-description/sharepoint-online-limits)
As a user I occasionally get an 503 error
503 indicates the SharePoint service isn't ready to handle the request. The common cause is that SharePoint is experiencing more temporary load spikes than expected. Unfortunately, this also impacts Filenize.