The underlying reason seems to be that the stationary lights require SM channels for casting shadows  from movable objects and they are assigned channels 0,1,2 by lightmass (or somewhere along the way, the preview channels anyway). 

The directional lights disable channel 3 for reasons to do with cascade blending (according to a comment), which leaves them without. The stationary lights have higher priority, and cannot be re-assigned by the channel allocator.


Steps to Reproduce

1. Build & run QA Game

2. Switch to forward shading

3. Restart

4. Create 3 stationary spot lights

5. Set the directional light to movable

6. Possibly also save and reload

7. The error should appear.

Have Comments or More Details?

There's no existing public thread on this issue, so head over toAnswerHub just mention UE-80743 in the post.

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ComponentRendering - Features Teams
Affects Versions4.24
Target Fix4.26
CreatedSep 24, 2019
UpdatedMar 19, 2020