I'm using both mods (belt config and miniloader) and maybe around factorio version 0.18.8 they stopped working, when I know they used to work together.

Everything looks good in a serpent.block. The belt speeds and miniloader speeds are all higher, but in-game the miniloaders are default speeds (15/30/35 items per second). I'm posting here and on miniloaders, because I don't know if either of you would have any idea what is wrong.

First of all, Schall Belt Configuration have NO special code about miniloaders. This mod (and extension) only apply values on REAL loaders, namely type loader and loader-1x1. Thus can work with mods like “Deadlock's Stacking Beltboxes & Compact Loaders”, “Space Exploration” and "Krastorio".

On the other hand, Miniloader is using DISGUISED loaders, which are type inserter in reality. (Can be read from its mod main page "How it works" section.)
I do not have the time to test in detail, but just take brief look into its code. My best guess is those invisible inserters are assigned a lower value than needed. Miniloader is using a "stepped function" on actual inserter speed. You can see from prototypes/entities.lua local function inserter_speed(belt_speed). I think it is the main cause.
Also note that recent game versions tinker with inserter basic mechanism, changing the "standard" inserter speed. Perhaps it affects as well. For example:

0.18.7 Bugfixes: Fixed that changing inserter pickup/dropoff through mods didn't work correctly. (81263)
0.18.9 Bugfixes: Reverted optimisation that caused inserters picking up items from belts to be slower.

Therefore, you may better go ask therax and devs on this issue. There is nothing to fix on my side.

Thanks.