Error: Failed to load messages

Hi - brand new user here

I migrated approximately 263,000 messages into an Inbox/Gmail Import folder via IMAP. Thunderbird can access and open all messages normally. Other folders work normally in the mailbox.org web interface, and I was able to access them for a while but now opening the Gmail Import folder consistently gives “Error: Failed to load messages”. The folder contains approximately 263,000 messages and has approximately 144,000 custom IMAP keyword assignments. The messages and keywords are accessible through IMAP/Thunderbird.

Is there a known IMAP issue stopping me viewing these?

yours
Tony Makos