Obstacle Avoidance does make an error in flying stark sun with Spark.
Suddenly it stops and cannot fly forward and one have to fly backwards
home.. Dear DJI please set sensitivity down, and make a manual option
to set custom sensitivity, when flying in sun and forests, where it often
blocks any flying and filming.

Have any others been blocked from forward flying with Spark in sun?

Happens all of the time, I just turn it off.

I would not recommend you fly toward the sun. Please read the user manual and avoid the following situation.

The sensors are highly sensitive to direct sunlight. Getting those kind of errors are common when you're looking directly into the sun. Like what others mentioned, you'd want to turn off the sensors if you plan to be in extreme lighting.