Update:
FYI This relates to a new installation on a new development website, but this seems to be the same problem we were experiencing previously (please see the other ticket). It is important to note that this was never resolved. However we have a workaround in place for the time being. It is very curious, but by making the asp cache directory unwritable the filter works fine! So only in this case does the plugin work correct (but we see the php errors appear in the log that we have an unwritable directory which is correct). If we deactivate and reactivate the plugin it recreates the cache directory and the plugin again does not work and has the 503 error.
Can you please provide support as we are stuck here. Thank you!
Marcel