The plot thickens. Did another test.

I made something, 1 min 20 long.

bass line , kick and hi hat. Bass Line is om2. ( just inserted the vst, and changed nothing )
Exported the track and no problem at all. :o
:P

Just did another export, 6min 57.

Instead of two om2 now 2 zynaddsubfx are playing the bass line.

Not one crack or stutter :)

( except of course the crackling in the samples, clearly recorded from vinyl )

Now maybe a test with om2 in a new project.
My first suspect were the samples too, but I noticed crackling and stutter when no samples were played, in the audio file processor. I had already tried a few thing, but then you asked: What are you doing different this time ? Then I almost instantly thought of that one vst. All other synth and vst,...
Looking forward to it, but is that not spelled acid ?
Pc seems fine, last render I did, was while I had multiple program open, and that render was fine. ( after removing and replacing that one vst for synth 1 ) Pc is also stable, so I don't think there is any hardware issue's . I might do some test, but this one vst, om2 does not seem to play nice when...
Could be interesting, I have no problem, when I listen in LMMS, only with exporting with this vst.

Edit:

Forgot to mention something, this vst seems to play actual recorded base guitar notes.
Notes do snap to the grid ?

When I record from my midi controller, were the note lands in LMMS, depends on the Q setting. ( and my timing)
Did a dirty test, with other programs open too. Deleted one track, and replaced on the other the bass guitar vst for synth 1. I was using that vst twice, maybe it does not like that ? I am almost afraid to cheer to soon, but this seems to solve the export problem. :D The only crackling I hear now is...
After some google, it seems to be another Linux os. So you could try the appimage for Linux. On a side note: LMMS supports most distro's, you can't seriously expect them to support every distro around. It works on Ubuntu, Mint, fedora, Arch, openSUSE and some more, those are the most used Linux Dist...
Thank you and will do. Also don't think its in those cables after some more thinking. I would have constant crackling ( and an unstable pc as you said) if that was the problem, and only have an issue exporting this track. Also tried changing the setting from sdl to alsa. That changed as expected not...