MV build 5 is one of builds that came after the Vx Ace 0.50a. Unstable and buggy. It was released just to show the technobrake's progress of porting the game content to the new engine.Actually I played most version since 50a (new save in 50a, 51a and MV build 5) and had same problem in every one (on 50a had some error in Japanese during that event)
NymPri 0.51 is an MV version of the game that represents the end of porting the game content to the new engine. As far as I remember, it wasn't the most stable version. If I recall correctly, this one was reported to have many bugs, so I avoided it and decided to wait for something more stable and, most importantly, much less buggy.
Strange thing with this error you mention for Vx Ace 0.50a; it's first time I hear about it. However the other two versions you mentioned are absolutely no surprise to me; bugs occuring after porting the game to the new engine are a pain for sure.Guess I'm doing something wrong since I have same problem in 3 totally different version, will have to try things out, thx for guide!
I wouldn't be surprised if it were because of the bugs that you couldn't improve your Yarizo relationship level in the MV build 5 and NymPri 0.51 versions.
I can only recommend you to get your hands on the most recent version. NymPri 0.53 has no story, however while playing it I didn't encounter any bugs that would crash the game yet.
I hope this guide will help you in any way possible to finally get what you aim to achieve.