just loaded my 2513 savegame after updating to 2521 and got it again
I had two unfinished buildings in that savegame and when I pause/restarted one of them I got the error
develop-2521 (x64)
...earth/services/server/build/scaffolding_manager.luac:49: assertion failed!
stack traceback:
radiant/modules/events.luac:68: in function <radiant/modules/events.luac:65>
[C]: in function 'assert'
...earth/services/server/build/scaffolding_manager.luac:49: in function '_trace_rblock_region'
...earth/services/server/build/scaffolding_manager.luac:33: in function '_add_region'
...th/services/server/build/scaffolding_builder_1d.luac:51: in function '_add_scaffolding_region'
...th/services/server/build/scaffolding_builder_1d.luac:57: in function '_update_status'
...th/services/server/build/scaffolding_builder_1d.luac:40: in function 'set_active'
...th/services/server/build/scaffolding_builder_2d.luac:43: in function '_update_status'
...th/services/server/build/scaffolding_builder_2d.luac:37: in function 'set_active'
...arth/components/fabricator/fabricator_component.luac:223: in function '_start_project'
...arth/components/fabricator/fabricator_component.luac:131: in function '_updates_state'
...arth/components/fabricator/fabricator_component.luac:127: in function 'obj'
radiant/modules/events.luac:15: in function 'self'
radiant/modules/events.luac:80: in function <radiant/modules/events.luac:78>
[C]: in function 'xpcall'
radiant/modules/events.luac:78: in function 'trigger'
radiant/modules/events.luac:97: in function '_update'
radiant/server.luac:19: in function <radiant/server.luac:17>
it’s the left one… the right one misses a roof since 5 days now and the left one hasn’t been started ever (both ordered at the same day)
update:
well at least they finish the buildings now 
