The WebGL message “Error: WebGL warning: uniform4uiv: Function used is incompatible with uniform type: 35669” is certainly an indication of a bug in UE4 WebGL backend. I would recommend making a minimal project (like you seem to have been doing already above?) to reproduce this and posting to the bug report section of this forum (Einen Fehler melden - Unreal Engine). A zip bundle + steps here might also help others to reproduce, and the UE4 devs to reach a bugfix.
The other WebGL messages like
Error: WebGL warning: drawElements: Framebuffer not complete. (status: 0x8cd7)
Error: WebGL warning: drawElements: Framebuffer must be complete.
can be ignored, they are present in all exports at the moment I think.
Also the CORS related message comes from the telemetry option, which you can disable in the project settings by choosing “End users do not send telemetry” (or something similar, I forget the exact UI option), though the warning message is harmless as well.
Any chance you could zip up and link your web export of the minimum problem repro somewhere? There is one JavaScript side trick I could try to see if this might be able to be worked around outside the UE4 engine.