Our server is almost full and we need remove space, we tried to check the folder size of our server and in this folder ( tmp/assets) there are a lot of files and increase the size all the time.

Is it possible to remove everything of this folder?

You should be able to remove everything except index.html. (but ALWAYS back-up first)

I cannot recommend that. Are you really talking about a complete server? Or just a webhosting package?

What exact version of LimeSurvey is used (look at the bottom of the LimeSurvey GUI)?

I cannot recommend that.

Why?

LS will recreate a lot of things with the next click inside LimeSurvey again. Doesn't seem to be the best place to save space for a longer timespan. And I don't know if any version of LS might be behaving a bit different in terms of /tmp/assets.

Yes, but it doesn't recreate everything. In my opinion, even the latest version does not flush this directory adequately (or, at all).