I think this issue is related to Right click kills the Expression node which should be fixed with 5.5.2 and 5.8.0. Version 5.5.2 was just announced a few hours ago, so maybe you can now update it directly from 5.5.1? 5.5 is the current stable version which will get bug fixes, but will not automatically update to 5.7 (the latest version, which will get the latest features every 6 weeks). See KNIME Analytics Platform Now Releases Every 6 Weeks on the recent changes on the update frequency, where this is explained in more detail.
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