Too few information.

1st of all it might be important that you remember, since when this problem occurrs.
Can you give the date and eventually time ?

Do you have any idea what might have changed between before, when it worked and after ?
- did a Windows Update occurr (check Windows Update history)
- did an VLC Update occurr ? Is perhaps the application configured to update automatically ?
- did something change to your setup/system:
-- do you use another USB or Fw port on your PC ?
-- did you add certain HW, like USB Hubs or Bluetooth or other adapters to your system ?
-- did you install new software or services to your system ?

Backup / Restore
Assuming an Windows / Software problem, is your system properly set-up, that you could restore a backup of the Operating System from the time, where you are sure that it worked, without loosing any of your user data ? The question behind this is
- do you have a working backup for OS and data
- are you experienced with it to backup and restore without any loss of user data ?
- are OS/programs and user data on separate partitions ?

And now some questions in regards to your systems setup etc ...

Which mainboard and CPU do you use, how much RAM is installed ?
What OS and build number ?

How do you connect the UFX:
- via USB or Firewire, what is the cable length ?
- is this a port on the mainboard or by an add-on card, which chipset ?

Over what devices do you listen ? Nearfield monitors or Phones?
To which UFX HW outputs are your monitors or phones connected where you hear the crackle ?
Which ASIO buffer size do you use ?

What VLC version ? Which sound driver does VLC use ?
Can you post a screenshot of your VLC setup in terms of which audio driver it uses ? ASIO or Windows audio via RME WDM devices ?

You say other applications do work: which applications are this and which audio driver do they use ? ASIO or Windows audio via RME WDM devices ? Maybe we can narrow by this whether this is maybe more an application related problem.

What are the surrounding conditions when the cackles happen ? Maybe system load ?
- How is the CPU load when the crackles happen ?
- Or do you see or hear special disk activity ? Maybe a concurrent backup Job  or Virus Scan ?

Is your audio PC connected to the Internet ? Via LAN or via WLAN adapter ?
If WLAN, does it make a change if you temporarily deactivate WLAN ?

Are you using eventually other wireless devices on your PC, i.e. bluetooth ... ?

Is your on board sound on mainboard deactivated or other question in that regards,
are you sure that the crackling in the playback comes from the UFX and not from another monitor
which might be connected to the soundcard of your PC ?

Ramses,
Thank you so much for the detailed answer. I tried it all except for a system restore that I just did. Now it works.
Thanks again!

I restored from a backup from a week ago.
The interesting thing is that no windows or Vlc updates had been done during that period.

I use the RME driver in VLC. No changes after the restore.

I really dont know why it is solved now. But I will let you know if I find out the cause.

I use windows 10 home.