I have tested this error on several PC, connection, account and emClient version. The error is easy to reproduce.
If I want to make space on my gmail account and i create local local folders 1 for each month, if I try to select from gmail inbox folder a whole month (example from 1-gen to 31-gen emails), right click, move to, local folder “gen.2019”, eMclient start to download the text and attachments of all emails to move in local folders, BUT if there are a lot of email, after a while emClient fail to download all messages with the error "[IMAP] unable to download the message (…) from the folder “[Gmail]/all messages”.
At this point, if i look on the local folder “gen-2019”, the folder is EMPTY, and if i look on the source folder “gmail inbox”, all the emails from 1-gen to the 31-gen are LOST!!
So, all the selected messages to move, are not found either in the local folder or in the gmail folder, they have all been deleted! They don’t even pass through the trash !!
This is very serious and systematically causes the loss of many emails. The only way to avoid this behavior is to move the emails by selecting a few at a time (for example a week at a time instead of a whole month)