If you double-click right on the individual song files (one at a time) they will download as single FLAC files with no zip archiving.
I just tried a few test downloads - both individually and with the zip feature. It just worked both ways and the download was fast.
I investigated the Mega page regarding downloading vs. importing to your cloud drive account.
If you click the import button and do not already have an account, you get an error message telling you that you need to log into your account or create one if you don't have one. As expected.
Double-clicking on a folder navigates to that folder's contents as expected and as noted.
Double-clicking on a file (FLAC or ALAC) downloads that single file with no zip archive. (I need to include a note about this function as it isn't completely intuitive!)
Clicking on the download as zip button makes a zip of the current directory (folder) you are in.
The dozen times I've tested the 'download as zip' function with the 5.1 folder it worked every time and the download took about 20 minutes.
I tried the 'import' function (I made a test account to import to). It prompted me to log into my account as mentioned. The transfer was instantaneous.
I clearly see a Mega page in the screen shot above with the quota errors. I don't know what sequence of events you need to do to get that error!
My only guess is it's an unsupported browser thing (like trying to use Internet Explorer - or "edge" they call it now - or perhaps Safari).
Mega lists both the stock Windows IE and Apple Safari as only "partially supported". They do appear to support Windows though. This isn't a Mac-only thing. It must be crashing with these browsers.
I recommend Firefox (which I'm thinking would make a lot of other problems go away as well for browsing in general).
I am trying to download now in Firefox logged on with a MEGA account, but keep getting File too big to be reliably handled in memory error. After installing the MEGA plugin the download is working for the 5.1 folder
edit: now getting an over quota error
edit 2: thought I was clever and removed the transfers, logged out and reopenen the folder without logging in, but still get the over quota error LOL. Works on IP address
OK. Hmmm... again then.
You have a Mega account.
Using Firefox.
Trying to 'import' to your account.
Are you running Windows? I know it's low hanging fruit and all...
Maybe Windows still needs extensions installed to deal with files over 2GB or 4GB (I forget the limit)? (The 5.1 folder is 3.28GB.)
All I can suggest is to either import them or download them song by song for a workaround and apologize for the inconvenience.
(Not trying to sound like an Apple ad either. My interest in them starts and ends with the pro computer line with no exceptions.)
Please let me know any more details in case it leads to something I can fix!