Raspberry Pi - fbi process blocking Teamviewer

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I might have a rare case, but at the moment I'm not able to find a solution for this problem. I've installed a RaspberryPi 3 in a local business to display images. There's a cronjob starting the bashscript on reboot. This script downloads some images of my FTP server and displays them via fbi: fbi -a -t 15 -T 1 -d /dev/fb0 -noverbose /home/pi/Slides/*.*

This part is fine and working. To access the raspberry remotely (SSH is blocked by the ISP router - no port forwarding possible) I've installed Teamviewer (gives me the advantage of accessing it with my mobile as well).

My problem: I can't connect via Teamviewer while the fbi process is running. But the process has to run all the time, so the customer can see the images. As soon as I kill the fbi process via SSH, I can connect

Is there any way of forcing the fbi process in the background or does anyone have another idea?