Ideas
Turns out, it's the same like you reported yesterday. I've merged your posts.
The LE on restarting a save on exam day (which was also Love Day) in the unmodded game looks similar if not the same as the LE on restarting a modded game on a regular school day that fell on Lottery Day. I suspect that this is due to the decision to use the holiday system to revamp the active school days, and the scheduled holiday is overriding the active school day, preventing it from being displayed/affecting school performance. So the fix for multiple teen sims not being able to participate in active school days broke active school days on holidays.
LE from modded game:
<?xml version="1.0" ?>
<root>
 <BetterExceptions>
  <BEversion>v3.14</BEversion>
  <WasBlank>False</WasBlank>
  <TuningLoadFinished>True</TuningLoadFinished>
  <BadObjectCC>True</BadObjectCC>
  <Advice>This is likely due to bad object CC. Check the Summary and Broken Mods section in BE Report.</Advice>
 </BetterExceptions>
 <report>
  <version>2</version>
  <sessionid>c451a613fa4c386c684167c9</sessionid>
  <type>desync</type>
  <sku>ea.maxis.sims4_64.15.pc</sku>
  <createtime>2025-06-05 05:52:20</createtime>
  <buildsignature>Local.Unknown.Unknown.1.115.216.1030-1.200.000.658.Release</buildsignature>
  <categoryid>indexed_manager.py:88</categoryid>
  <desyncid>c451a613fa4c386c684167c9</desyncid>
  <systemconfig/>
  <screenshot/>
  <desyncdata>
   [tingyul] Exception raised while trying to restore career event. (TypeError: IndexedManager keys must be integers.)

   Traceback (most recent call last):

  

   File "T:\InGame\Gameplay\Scripts\Server\areaserver.py", line 130, in wrapped

   File "T:\InGame\Gameplay\Scripts\Server\areaserver.py", line 126, in finally_wrap

   File "T:\InGame\Gameplay\Scripts\Core\sims4\utils.py", line 157, in wrapper

   File "T:\InGame\Gameplay\Scripts\Core\sims4\utils.py", line 175, in wrapper

   File "T:\InGame\Gameplay\Scripts\Server\areaserver.py", line 383, in c_api_client_connect

   File "T:\InGame\Gameplay\Scripts\Server\zone.py", line 611, in do_zone_spin_up

   File "T:\InGame\Gameplay\Scripts\Server\zone_spin_up_service.py", line 1740, in update

   File "T:\InGame\Gameplay\Scripts\Server\zone_spin_up_service.py", line 732, in on_enter

   File "T:\InGame\Gameplay\Scripts\Server\careers\career_service.py", line 359, in create_career_event_situations_during_zone_spin_up

   File "T:\InGame\Gameplay\Scripts\Server\careers\career_base.py", line 2806, in create_career_event_situations_during_zone_spin_up

   File "T:\InGame\Gameplay\Scripts\Server\careers\career_event_manager.py", line 523, in create_career_event_situations_during_zone_spin_up

   File "T:\InGame\Gameplay\Scripts\Server\careers\career_event_manager.py", line 386, in _start_scorable_situation_from_tuning

   File "T:\InGame\Gameplay\Scripts\Server\careers\career_event_manager.py", line 395, in _scorable_situation_setup

   File "T:\InGame\Gameplay\Scripts\Server\situations\situation_manager.py", line 1115, in register_for_callback

   File "T:\InGame\Gameplay\Scripts\Server\indexed_manager.py", line 88, in __contains__

   TypeError: IndexedManager keys must be integers.rtim=0
Client Session Time: 42.16
 

   Num Save Errors: 0
 

   Num Load Errors: 0
 

   Current Game State: 0x96d01090
 

   LoadingScreenActiveFlag: 1
 

   Origin Version: 13,477,0,5990
 

   Modded: True
 

   SystemInfo: Windows 11 10.0.26100 GPU: NVIDIA GeForce RTX 3060 Laptop GPU  32.0.15.6607, GUID: 3570AB96-EDF0-11EF-8AAF-D575D18A1133
 

  </desyncdata>
 </report>
</root>
I don't know why the Better Exceptions section mentions bad CC, I don't have any CC (just mods) and the BE-ExceptionReport.html doesn't identify any broken mods/CC.