Update guzzle to d8 latest
I discovered that there was an incompatibility between the guzzle version in Omni vs Civi. It is an error handling issue so it was showing up in the fileExists function in one of our status checks & is fairly low impact.
On digging it seems the issue was due to Omnipay having guzzle 6.5.x & CiviCRM having 6.3.x & the Omnipay one getting picked up for a core civi call but then picking up one class from the core civi one.
Obviously composer & extensions continues to be a challenge but here I think the immediate fix is to update CiviCRM Guzzle to the one that ships with latest Drupal 8 - currently version": "6.5.4",`
For now I'm downgrading Omnipay guzzle to match current CiviCRM