Apparently quarantined file cannot be restored

Dear Community,

After doing a full system scan of my computer, but cancelling half way through since it was slowing to a crawl after a few hours, one of my Outlook pst-mailboxes appeared to be missing. The slowing down was as far as I could tell not related to the mising mailbox, since it was scanning some texture files of a game. The mailbox was about 8 GB large. At the same time, the free space on the C: drive (the apparent location of the quarantine directory) was significantly reduced (the mailbox was on the D: drive), so it seems that the file was indeed quarantined.

However, when I look in Bitdefender and go to security -> quarantine, no files are mentioned as being quarantined, so none can be restored. When I look in C:\ProgramData\Bitdefender\Desktop\Quarantine directory, a BDQ-file of 8GB has appeared (together with a REF file), with a date/time stamp during the scan, so it seems this would be the mailbox-file. I tried to change the name to PST, but that does not work, so I assume it is encrypted.

How can I restore my mailbox? I do not have any reason to believe that there was any malware present, and either way I would want to get the non-infected emails back.

Kind regards,