Neighbor is encroaching on the property

Actually, I do not want to start any trouble

I would not speak of "starting" if your description is accurate. The other person already started.

Last time, politely but with the necessary firmness, demand immediate clearance and then act accordingly.

+1

In my experience, it is - symbolically speaking - important to quickly mark your territory. Later, you can always back down on minor issues. Otherwise, you will be the pug at the place forever.

Therefore, from me as well: a final request that the place must be vacated within 36 hours.

He always said he would take care of it, but nothing happens.

Actually, I don't want to start a fight

Don't see yourself as a troublemaker. Do it as described above: oral warning with written emphasis in hand – the written form should maintain proper form, yet you can write "after several oral requests"...

Maybe the colleague is a bit chatty; after a neighborhood beer, which you offer him during the property development when you meet him, things might smooth over again.

after a neighborhood beer, which is offered to him during the property development when you meet him, the waters can be smoothed again

There is absolutely nothing against inviting your neighbor, even this one, for a beer! Still, in my opinion, it is not up to eisbaer_oz to smooth things over. This would have to be done by his neighbor if he is interested in a reasonable relationship. If one is aware of this, one may still happily leave gifts at the neighbor's door (the smarter one gives in) out of gratitude that he finally refrains from storing his stuff on eisbaer_oz's property.

We sent him a registered letter today and set a deadline until the 10th, with the threat of taking further measures because he is obstructing us in our further work. We will see. Up to this point, I already thank you for your advice.

Ultimately, it would have been simple to discuss beforehand with the TE where and for how long he may store material on his property. Common decency demands that. It’s always a shame that such measures have to be taken. Hopefully, the waters will calm down, I wish you much success. Sent with the handheld device