Thanks for your reply but i solved it just now.
For everybody experiencing the same problem i'll show you how i did it:
In the WebUI i have:
user: transmission (uid 1005)
group: transmission (gid 1013)
In my transmission jail however i have the same user/group but with different uid/gid:
user: transmission (uid 921)
group: transmission (gid 921)
So i had to change the uid/gid inside the jail to match the freenas uid/gid for the user/group transmission.
Stop your transmission service.
login as root via ssh and do the following:
Code:
[root@freenas] /mnt/pool1# jls
   JID  IP Address	  Hostname					  Path
	 3  -			   syncthing_1				   /mnt/pool1/jails/syncthing_1
	 6  -			   transmission_1				/mnt/pool1/jails/transmission_1
 
in my case the jail number is 6, so to get inside the jail do:
Code:
[root@freenas] /mnt/pool1# jexec 6 tcsh
root@transmission_1:/ #
 
a listing of the filesystem looks like this
Code:
root@transmission_1:/ # ls -al
total 173
drwxr-xr-x  18 root	wheel		   22 Jan  7 17:48 .
drwxr-xr-x  18 root	wheel		   22 Jan  7 17:48 ..
-rw-r--r--   2 root	wheel		  966 Mar 25  2016 .cshrc
drwxr-xr-x   2 root	wheel			4 Jan  8 18:03 .plugins
-rw-r--r--   2 root	wheel		  254 Mar 25  2016 .profile
-r--r--r--   1 root	wheel		 6197 Mar 25  2016 COPYRIGHT
drwxr-xr-x   2 root	wheel		   47 Mar 25  2016 bin
drwxr-xr-x   8 root	wheel		   50 Mar 25  2016 boot
dr-xr-xr-x  11 root	wheel		  512 Jan 10 07:25 dev
drwxr-xr-x  23 root	wheel		  107 Jan 10 07:25 etc
drwxr-xr-x   3 root	wheel		   52 Mar 25  2016 lib
drwxr-xr-x   3 root	wheel			5 Mar 31  2016 libexec
drwxrwxr-x+  5 921  921	 5 Jan  8 14:45 media
drwxr-xr-x   2 root	wheel			2 Mar 25  2016 mnt
dr-xr-xr-x   1 root	wheel			0 Jan 10 07:32 proc
drwxr-xr-x   2 root	wheel		  146 Mar 25  2016 rescue
drwxr-xr-x   2 root	wheel			7 Jan  8 18:06 root
drwxr-xr-x   2 root	wheel		  132 Mar 25  2016 sbin
lrwxr-xr-x   1 root	wheel		   11 Mar 25  2016 sys -> usr/src/sys
drwxrwxrwt   6 root	wheel			6 Jan 10 07:25 tmp
drwxr-xr-x  15 root	wheel		   15 Jan  8 18:03 usr
drwxr-xr-x  25 root	wheel		   25 Jan 10 07:25 var
root@transmission_1:/ #
 
change the uid to match the freebsd uid (in my case 1005):
Code:
pw user mod transmission -u 1005
change the gid as well (in my case 1013)
Code:
pw group mod transmission -g 1013
 
you have to edit the /etc/passwd file as well (i guess not sure about that one), and you should do it with vipw, so it looks like this:
Code:
transmission:*:1005:1013::0:0:Transmission Daemon User:/usr/pbi/transmission-amd64/etc/transmission/home:/usr/sbin/nologin
 
restart your jail, start the transmission service and you should be golden.