The problem is fixable, but there is a nuance. Unfortunately, you'll have to lose all, most or some of your progresses on downloaded levels. Don't follow this tutorial if you are not ready to lose your custom level progresses.

Well, here comes the main part.

!! IF YOUR MARBLE IT UP ULTRA GAME IS OPEN DON'T FORGET TO CLOSE IT BEFORE DOING ANY OF THE STEPS BELOW !!

At first, open the folder with Marble it up Ultra custom levels. It is located in folder where all the Steam stuff is - steamapps - workshop - content. For me the path looks like this - D:\SteamGames\steamapps\workshop\content.

https://gtm.steamproxy.vip/ugc/2317729541143483284/D04BA3DDFCA87103F84872D790024CC3D256F98C/?imw=256&&ima=fit&impolicy=Letterbox&imcolor=%23000000&letterbox=false

The folder must look something like this.
When you're there, folders are named by bunches of random numbers, and you'll have to search for the one with Marble it Up Ultra levels. If there is only one folder in content folder it means you haven't download any workshop stuff in other Steam games and don't need to search for the needed folder. If you do have more than one folder in content folder you can use my method:
1. Type in search name of any downloaded level
2. Put your cursor on the path so the full path will be seen
3. Remember the shown folder, close search function using cross in search bar, and open the folder. Inside every folder in the needed folder there must be some of your download level.

https://gtm.steamproxy.vip/ugc/2317729541143506992/59B170B5CF370252CB4EF948C13E6ADB09FDAD30/?imw=256&&ima=fit&impolicy=Letterbox&imcolor=%23000000&letterbox=false
https://gtm.steamproxy.vip/ugc/2317729541143511178/A2BCB5C2E628DB6861EC59E45178AB2A5988E15D/?imw=256&&ima=fit&impolicy=Letterbox&imcolor=%23000000&letterbox=false

Here you are. Now somewhere (for example on your desktop) create new folder. Then copy all the folders in the marble it up custom levels folder and paste in new folder. Then delete all the folders in the original marble it up custom levels folder.

https://gtm.steamproxy.vip/ugc/2317729541143540615/D1B4A2AD9649E7A8D46CFBF786009D6947923550/?imw=256&&ima=fit&impolicy=Letterbox&imcolor=%23000000&letterbox=false
https://gtm.steamproxy.vip/ugc/2317729541143543182/8301F356D3AB9F86A062BED62F4EE7ECC30BA9A8/?imw=256&&ima=fit&impolicy=Letterbox&imcolor=%23000000&letterbox=false

Now add one (any that you want but just one) folder from the new folder and copy-paste it in the original Marble it up Ultra custom levels folder.

https://gtm.steamproxy.vip/ugc/2317729541143558549/082896DD02630D914C62E996C9CA1229631B83DD/?imw=256&&ima=fit&impolicy=Letterbox&imcolor=%23000000&letterbox=false

Here you are! The level is loaded.

https://gtm.steamproxy.vip/ugc/2317729541143580812/B17EBB3F62B038BBD5AD6F9BC95DE5BFF9CF399F/?imw=256&&ima=fit&impolicy=Letterbox&imcolor=%23000000&letterbox=false

Now the game is not overloaded and you can subscribe to new levels and they will add. Unsubscribe and subcribe if you're subcribed to the level but it doesn't appear. If you're lucky enough when you add some levels back you won't lose the progresses.

https://gtm.steamproxy.vip/ugc/2317729541143603160/2C5BE47D04E18BC0B29A20F6B08C8362AF8AE59C/?imw=256&&ima=fit&impolicy=Letterbox&imcolor=%23000000&letterbox=false


!! DO NOT SUBSCRIBE TO MANY LEVELS AT ONCE. IT WILL OVERLOAD THE GAME AGAIN AND CUSTOM LEVELS BUTTON WILL DISSAPEAR. I RECOMMEND SUBSCRIBE TO ONE LEVEL AT ONCE BEFORE LOADING THE GAME !!

Do not panic if the levels or custom levels button don't load immediently. Reload the game, wait a bit, and it will work. You can add previously downloaded levels one by one too. Thanks for reading!