What characters do you have to remove in order for this to work in PlantText.com? Can you give me a minimum working example so I can look at the differences between working and not working, to see what syntax is causing the problem?
PlantText uses the PlantUML web service program, which requires sending the deflated version of the markup as a GET HTTP request. So, I tried running this example directly against their PlantUML web service and it did not work either. This means either the PlantUML web service has not been updated to a version that supports the feature you are using, or perhaps it is a problem with just the web service version or sending over HTTP.
I would try using the PlantUML jar file at the command prompt and see if it works in that more direct way. Let me know if you have already tried that and know the answer. If that works, then you need to post a bug report to the PlantUML folks and they will make it work. Again, PlantText is using their PlantUML web service so that is the only way to get it working for you.