When the CPU has less than 5 logical cores the game will only assign 1 thread to the logic worker group.
There is a bug where the logic worker group gets deadlocked when loading mods with a single thread.

Workaround:

1. Open the game to the Main Menu

2. Go to Options -> Game -> Worker Threads

3. If you're having the mod downloading issue I would expect this value to be 4 or lower. If it is not 4 or lower then please let me know and don't keep following these instructions.

4. If the worker thread slider goes up to 5 (or more)
   a. Set the slider to 5
   b. Press okay and restart the game
   c. Try to load a modded world.
   d. If you still get stuck on loading mods or checking for mod updates, let me know
   e. Stop following these instructions.

5. Otherwise, if the worker thread slider doesn't go up to 5....
   a. Set the worker thread slider to something different than the current value.
   b. Exit out of the game
   c. Press Win+R, write %AppData%\MedievalEngineers\, then hit enter
   d. Open MedievalEngineers.cfg using Notepad (or your text editor of choice)
   e. Find the part that looks like <item> <Key xsi:type="xsd:string">WorkerThreads</Key> <Value xsi:type="xsd:string">NUMBER</Value> </item>
   f. Edit the NUMBER to be 5
   g. Open the game and try to load a modded world.
   h. If you still get stuck on loading mods or checking for mod updates, let me know.
   i. If you open the game options again after making these changes it will likely revert to a non-working value and you'll have to follow these steps again.