When I make a tf portal and start jumping in and out after about 10 Times i get teleported to y=4 in tf and when i go back to the overworld, some chunks are swapped with tf chunks, the portal is solid (i can walk over it) and there is a new portal above, that has the intended visual effects, but doesn't tp me anywhere

Please attach the full game log.

dont know which one so ill send u all.zip

this time i got tp'd very high in the overworld, the new portal was in tf, neither of them were solid and also these blocks appeared

Looks like it's the same error as #356 which I've yet to do a deep dive on (wasn't able to figure out the cause from just looking at Betweenlands' source).

Where can I find it?

In your .minecraft folder there should be a logs folder containing a debug.log. Note that it only contains data from the last time you started MC, not any previous runs.