Yahoo mail can't start new thread after about 1,000 emails downloaded

[I33Buckler]

The following message appears after downloading about 1,000 emails from Yahoo IMAP.
As there are nearly 100,000 emails on the server I would like to be able to download the lot instead of having to run about 100 instances of OfflineIMAP to obtain all emails.

ERROR: while syncing Inbox [account main]
can't start new thread
ERROR: ERROR in syncfolder for main folder Inbox: Traceback (most recent call last):
File "/usr/share/offlineimap3/offlineimap/accounts.py", line 661, in syncfolder
remotefolder.syncmessagesto(localfolder, statusfolder)
~~~~~~~~~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "/usr/share/offlineimap3/offlineimap/folder/Base.py", line 1176, in syncmessagesto
action(dstfolder, statusfolder)
~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^
File "/usr/share/offlineimap3/offlineimap/folder/Base.py", line 1000, in __syncmessagesto_copy
thread.start()
~~~~~~~~~~~~^^
File "/usr/share/offlineimap3/offlineimap/threadutil.py", line 205, in start
ExitNotifyThread.start(self)
~~~~~~~~~~~~~~~~~~~~~~^^^^^^
File "/usr/lib/python3.13/threading.py", line 975, in start
_start_joinable_thread(self._bootstrap, handle=self._handle,
~~~~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
daemon=self.daemon)
^^^^^^^^^^^^^^^^^^^
RuntimeError: can't start new thread

This is the configuration with the user details anonymised.

[general]
accounts = main
maxsyncaccounts = 1

[Account main]
autorefresh = 0.5
quick = 10
localrepository = Local
remoterepository = Remote

[Repository Local]
type = Maildir
localfolders = ~/main
createfolder = true

[Repository Remote]
usecompression = no
maxconnections = 1
type = IMAP
remotehost = export.imap.mail.yahoo.com
remoteuser = main
remotepass = password
createfolder = true
ssl = yes
sslcacertfile = OS-DEFAULT