as per the error, did you enter your CMS Secret Key? It can be found in CMS Settings page and need to be provided along with CMS address on the player connection form.
I have the same issue but my problem is I am not seeing any key in the CMS setting page they are all blank and if I tried to put in a key it doesnt save. Any advice will be very helpful. If this works then I can buy the license for xibo android client.
I am using 1.7.3 version for CMS and latest Android client.
Could be good to use 1.7.10 or ideally 1.8.9 if you are also using 1.8 series player and want to use all new features in that series.
CMS Secret Key should be on the CMS settings page on the configuration tab, alternatively you can look for it in mysql, however if it’s empty in settings and you can save it then that would imply some other issues with your installation.
I think I’d suggest upgrade to 1.7.10, and if the will issue persist then please go through the Report Fault Wizard and send the relevant log file to me via private message.