First off, tell us what package you are installing, WITHOUT putting any link here.
(I am assuming it is not from IKM's website)..
Just the name and date will do.

I can not say for sure what is in that folder on a clean install.
I looked inside my archived "STEAM\Omnisphere\Settings Library\Multis\ folder,
(the one provided by Pirate), and it has:

A "3rd Party Libraries" folder, few 3rd party folders, the "Factory" and "User" folders.

The contents of the "Factory" folder is a file "Omnisphere Library.db"
The contents of the "User" folder is an EMPTY "My Category" folder, and 3 files:
1) 1st Bass Piano_omn.mlt_omn
2) Glory to God Alone.mlt_omn
3) Preferences.xml

OK... you told us a problem and then you say the problem stops.
When you say,"the error stops", what do you mean?
Are you now using Omnisphere without a problem?
=====================================

Sigh ...
If you look inside the "Current Categories.zip" You will find a file called "PLEASE READ.txt"
Please read:
---------------------------------------------------------------------------------------------------------------------------------------------------
- The "Current Categories" folder contains the most up-to-date categories used in Spectrasonics Factory Patch Libraries.

- If you wish to save your own Patches in categories that match the ones currently used by Spectrasonics,
you can rename the "Current Categories" folder to anything you like and save your Patches to these new folders.

- The original User folder will still be available to you in the Directory.

- You may also elect to transfer the Patches from your "User" folder categories to the new ones,
delete the original "User" folder and rename "Current Categories" to "User.”
-----------------------------------------------------------------------------------------------------------------------------------------------------

In the same way as I explained to you in PM a couple of days ago about the STEAM folder not having write permissions,
do a search using relevant words and see what comes up.
(Remember the first result we got for that was a bunch of threads on this very site when I used:
"The STEAM folder does not have write permission. Try running as Administrator." ?)
Do that with this problem. I used :"omnisphere missing multis folder">
I got scrolling one click on my mouse wheel brings me to a bunch of threads on THIS SITE regarding that issue:
https://www.google.com/search?q=omn...yosBHTCBAA8QrQIoBDAQegQIGRAO&biw=1182&bih=821

People get annoyed when others won't even do the bit of searching that MIGHT actually teach them something about the software.
As I said in my PM regarding doing searches in this way:
"You should at least need to know that much to be running cracked software.
If you can't figure that out through searching, I suggest giving up."

I provided some results to read through, but PLEASE do some searching yourself.

Certainly someone may be able to give a fast answer about this particular issue (I have not had the issue recently so I do not recall) , but if you are going to be using software that does not come with "support', the cost is sometimes investing in learning more about computers, settings, and ..believe it or not, our own human tendency to THINK we have followed instructions properly.
However "clean" your install, it may be that you did make a mistake on install.
If you have not used that one, just to give you confidence, it has the most EXPLICIT step-by-step instructions to follow and if you do follow them it WILL WORK. I even downloaded that and reinstalled myself just because I liked the organized way it is done.
I did not need the STEAM library at the time because I had my own that worked fine, but keep it as reference.
Unless you have some other computer issues it will work - Pirate gives instructions in the first steps to avoid the most common issues.

I will PM with link for a multi folder without the 3rd party stuff in case it helps.