Rendering out a particle sequence appear all the parts without any particle/object are cut out leaving a white band with variable height



There is any Mantra parameter setting to do so.

What are you loading the exr into? Is that photoshop or something? Photoshop is garbage for editing images generated from vfx software. I recommend using Nuke, or converting with Houdini's cops to a format Photoshop can understand first.

Using Affinity Photo [affinity.serif.com] (Photoshop-like), but also HitFilm Pro [hitfilm.com] (AfterEffects-like) does have problems.

Since the upper part is transparent and his size depend from the lower content the images are considered from HitFilm as variable size instead of standard 1280x720, and it's impossible to produce a video.
Discovered Davinci Resolve is able to produce it but with discontinuities (some gaps).

Also the images are really dark respect the Houdini Render view.

Perhaps the software you're using (Affinity Photo and HitFilm Pro) do not support the data window feature in EXR. You could try disabling “Enable EXR Post Processing” (add it to mantra ROP via Edit Parameter Interface) and see if the problem persists.