Eudora's mailbox errors almost always trace to its file based mailbox storage having a problem, since Eudora stores each mailbox as a plain mbox file with an index and the error moving messages usually means one of those is corrupt or constrained, so the causes and fixes:

The index corruption, most common: Eudora keeps a table of contents index, a toc file, alongside each mailbox mbox file and when the index and the actual mailbox contents fall out of sync, moving messages errors. The fix is rebuilding the index, delete or rename the mailbox's toc file while Eudora is closed, and Eudora rebuilds it from the mbox on next open, resolving the mismatch. This is the classic Eudora repair for mailbox operation errors.

The mailbox size limit, the old constraint: Eudora's mbox mailboxes have historically had a 2GB size limit from the old file format, and a mailbox approaching or hitting it errors on adding more messages, moves included. The fix is splitting a bloated mailbox, moving older messages into a new separate mailbox to bring the original under the limit, since the format simply cannot hold more.

The corrupt message or mailbox, the deeper case: a specific corrupt message can error the whole move, isolated by moving messages in smaller batches to find the one that fails, then handling that message alone. A genuinely corrupt mailbox file may need Eudora's own compact mailboxes operation, Special, Compact Mailboxes, which rewrites the mailbox cleanly and often clears errors from accumulated cruft or deleted message gaps.

The order and the backup: compact mailboxes first as the gentle general fix, rebuild the toc index if compacting does not resolve it, check the size limit if the mailbox is large and isolate a bad message by batching if a specific mailbox resists. Critically, back up the Eudora data folder first, since these are file operations on an old format and a copy means a failed repair costs nothing, especially given Eudora being discontinued makes the mail worth preserving to a modern format eventually anyway, the mbox files being directly importable elsewhere when that day comes.