5.17.10 and KiTTY 0.74.4.13 works fine with the same key. 5.19 does not for some reason. Other sessions using a different key seem to work fine.
Debug log:
. 2021-06-25 16:25:55.597 Offer of public key accepted
! 2021-06-25 16:25:55.597 Authenticating with public key "ed25519-key-20200925"
. 2021-06-25 16:26:03.035 Attempt to close connection due to fatal exception:
* 2021-06-25 16:26:03.035 Authentication log (see session log for details):
* 2021-06-25 16:26:03.035 Using username "root".
* 2021-06-25 16:26:03.035 Authenticating with public key "ed25519-key-20200925".
* 2021-06-25 16:26:03.035 (ESshFatal) Out of memory
. 2021-06-25 16:26:03.035 Closing connection.
. 2021-06-25 16:26:03.035 Selecting events 0 for socket 3656
* 2021-06-25 16:26:03.044 (EFatal) Out of memory