All was good after stopping the 1.20.1 server. After starting 1.21.1 existing photos are white.

Exposure does not support upgrading the world from 1.20 to 1.21. Too much internal changes happened between those versions.

I don't understand, il's only about mod data, no ?

Is there any documentation on .dat format of exposures ?

No. But it's not too complicated:
https://github.com/mortuusars/Exposure/blob/1.21.1/common/src/main/java/io/github/mortuusars/exposure/world/level/storage/ExposureData.java