Solved
Thingworx Mashup doesn't load in Design Mode
ThingWorx 9.3.6-b1323

ThingWorx 9.3.6-b1323

I've had it a few times too, with different causes.
The following has helped me every time:
1. Clear browser cache
2. If that doesn't help, maybe check the expiration dates of the stored licenses, under subsystems - licensing subsystem settings - check the expiration date here and update it if necessary
3. if that doesn't help, restart Tomcat
Doing these three things has always helped, at least for me.
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