I had the same issue, and wrote to the support. They responded quickly, and fixed the issue for me. Thanks a lot to them for their reactivity and for the fix!
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In case anyone is interested in the technical details:
It seems that this error occurs when editing the ‘extend_builder_theme’ setting directly in PHPMyAdmin. This corrupts the format of some quotes; and later Colibri fails to unserialize the data and crashes.
The fix is to repair the serialized data. This can be done using the script in the following gist: https://gist.github.com/kemenaran/a9a52d516ac556a57058fab7dcc8f92b
