Hi.
I am experiencing the same thing as nielsg99 has described.
I am using Turnkey MineOS.
I have checked that my Tekkit server is running the correct JAR, which it is.
I had exactly the same problem with creating a Voltz server.
When I tried to place a mod item, it would just vanish.
With a bit of searching I found a post that suggested to add some other files.
For Voltz, I added the following files to the LIB folder:
bcprov-jdk15on-148.jar
scab-library.jar
Having added both these files at the same time, I do not know if adding one, the other or both of these files fixed the problem. After adding these files, the problem of placing down objects was fixed.
I was thinking that this problem seemed very similar to my Tekkit one, but not finding it so similar with respect to the files in the server folders.
I am hoping that by posting this I may find further information on how I may fix this problem.
Any comments will be appreciated.