Hi, all!
I would upgrade our OJS 3.1.2.1 to the most errorfree, stable version 
Can anybody advise me ?
Hello @Spaik,
The most recent, stable release packages were announced recently on this post: OJS/OMP/OPS 3.3.0-3 and 3.2.1-4 Released
You may want to take a look at the Release Notebooks here: Release Notebooks - OJS/OMP - to get a sense of the features of the different versions, and what might be best suited to your needs.
Best regards,
Roger
PKP staff