Skyrim Skill Uncapper(V1.1.0) uses code from skse with bug that was fixed on 28 Dec 2020
https://github.com/ianpatt/skse64/commit/4a1e1219578890fb8cfb077daf08b386855ec62f
Depending on the memory address where the Uncapper was loaded, initialization may fail. The address depends on different factors, so you can be lucky or unlucky(Linux users are very unlucky as I learned from investigation). There are rituals for changing the address, like renaming a file so it is loaded first. But there is always a random factor(Although the sooner dll is loaded the less random address will be). Each time you start the game, Uncapper will either work or not. I was checking SkyrimUncapper.log and restarting the game until initialization was successful, until my patience ran out and I began to search source of the problem. If you are confident in your luck or don't trust strangers, use original. When Uncapper is not initialized you will not lose skills above cap, they will just freeze.
I updated Uncapper code and compiled dll. It seems to work for me. If somethting does not work for you, it is your problem not mine, poke author of the original to update.
While testing 1.6.640 got this bug once. So (fixed) in the title should be (partially fixed). Skse will show message if Uncapper failed to initialize, so restart Skyrim until there is no message.


Troubleshooting steps.
1. Check that you have Uncapper profile. SkyrimUncapper.ini must be in same directory as SkyrimUncapper.dll. You either need to download one or edit one, that will be created after first game start. If you use MO2 it puts files created during game into the "Overwrite" folder, search ini there. Open SkyrimUncapper.ini and check that [SkillCaps] and [SkillFormulaCaps] are changed from 100.
2. Open directory with logs
%USERPROFILE%\Documents\My Games\Skyrim Special Edition
It will automatically redirects to something like
C:\Users\your_username\Documents\My Games\Skyrim Special Edition
3. Check that skse loads dll.
3.1. Open SKSE\skse64.log
3.2. Find strings that mentions SkyrimUncapper.dll.

checking plugin F:\Steam\steamapps\common\Skyrim Special Edition\Data\SKSE\Plugins\\SkyrimUncapper.dll
plugin F:\Steam\steamapps\common\Skyrim Special Edition\Data\SKSE\Plugins\\SkyrimUncapper.dll (00000001 SkyrimUncapper 00000001) loaded correctly (handle 8)

If you can't find it or it tells something other than "loaded correctly" then you somehow messed up installation.
My Skyrim is installed in "f:\Steam\steamapps\common\Skyrim Special Edition", file path is "f:\Steam\steamapps\common\Skyrim Special Edition\Data\SKSE\Plugins\SkyrimUncapper.dll"
This process is kinda hard to mess up.
4. If dll is loaded, check that it initialized successfully.
4.1. Open SkyrimUncapper\SkyrimUncapper.log
4.2. Go to the end of file. If you see

couldn't allocate trampoline, no free space before image
couldn't create codegen buffer. this is fatal. skipping remainder of init process.

then unfortunately this fixed uncapper is not so fixed. Tell other users about that. If you see

Init complete

but sure Uncapper is not working then I don't know how this could happened. Check log files timestamps that they were changed when you started game and not leftovers from good old days when everything worked.
5. If you got here, it is very bad. Look for someone who will help you. It will not be me. For now I only do minimal bug fixes for Anniversary Edition update while SkyrimSE.exe is not stable.
Go to "Bugs" tab and create bug report. The minimum information you must provide includes content of skse64.log and SkyrimUncapper.log. Describe how did you install Uncapper. Do you have SkyrimUncapper.ini in same directory as SkyrimUncapper.dll?
At least other users will know what problems to expect. And if you care about others don't use generic titles like "Not working". For original bug good title would look like: "couldn't allocate trampoline" error during initialization.


Hints how to compile my sources(Because it took me few minutes to update original and whole day to compile it):
Skse uses Microsoft Visual Studio dialect of C++. Uncapper uses skse code and continues this practice, so you will need Visual Studio. Original used VS2015 and I used VS2019 so any not ancient version is probably fine.
Content of archive:

• common directory - Common classes used by the script extenders. You can get last version by: git clone https://github.com/ianpatt/common
  

• SkyrimUncapper directory - I took content of the original(SkyrimUncapper and Relocation directories) and updated skse. And mixed it together. No promises that I didn't create new bugs during this process.
  

• CMakeLists.txt - I don't like Visual Studio, so I wanted to create project as cmake file that I will compile in Qt Creator only using Visual Studio compiler and linker. But I failed. Still this file must be used to generate project for your version of Visual Studio. While creating project you need to specify platform for generator as "x64". I compiled release version of generated project without any errors(debug version will fail).