Hi, I've had the same problem in Ubuntu for a while now, at least since the 1.998 version of Webmin. There appears to be a bug in the perl code in edit_part.cgi (Webmin fdisk module), where the $ui_buttons_code variable is reassigned instead of being concatenated. Same error in edit_disk.cgi.
Same behavior here. Downgrade to version 5.1.0 (5.2.0 has another problem: Thunderbird requires password at every startup). I'm using imap, smtp, caldav & carddav, EWS protocol. MS Windows 7 x64 - JRE 1.8.0_221 x64 - Thunderbird 60.8.0
This problem seems to be solved with the latest upgrade. See https://www.raspberrypi.org/forums/viewtopic.php?f=28&t=222475
Unable to open folder when @ in directory name