I had a similar problem and I solved it by changing the permission.
Try to change the permission in the thing to everyone.
I am also not an expert, but maybe this helps
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I still have the issue and I can't share any data, because I am not able to open the Composer.
We are already in contact with PTC and try to solve the issue. I can share the solution, when we are finished.
Thanks for your reply,I still have issue for this problem,my ThingWorx Platform version is 9.1 and the MQTT extensions is MED-61334-CD-091_F000_MQTT-4-0-1-Extensions.zip.
I want to use MQTT function to publish my Things properties data when the data changes，but I can‘t find the useful method to work it out.