It was discussed in PM and fake 5.3.8 works well too. So I attached that file to my original post just in case there are more people who want to downgrade their cameras to 5.3.x.
I'm glad it helped you!
I just found a way how to downgrade 5.5.0 to 5.4.x (it worked on DS-2CD2142FWD-I at least).
In short, use hikpack to change 5.4.x's version to 5.5.0 and then upload it via tftp.
Although the file version is changed to 5.5.0, the Web UI says it's V5.4.x and allows to upload any 5.4.x through...