Attachments containing more than 21 Japanese characters are not accepted.

[JPcube]

Hello, I'm a Japanese IT professional working on a Roundcube project for the first time. When I create an email and save it as a draft with an attachment containing more than 21 Japanese characters, the file name is displayed, but it shows as 0 bytes, and the email subject is not displayed. When a received email contains an attachment containing more than 21 Japanese characters, the email list displays the subject and a paperclip indicating the attachment, but when I open it, I get an error message saying [oops]. My IMAP server is the e-post mail server. Connecting to the e-post mail server via IMAP with Thunderbird works fine. Also, connecting to Devcot as a test works fine. I'd appreciate any advice on how to resolve this issue.