Hi DavidShare ,
This error is due to an incompatibility with third-party software.
We (and many other plugins) use the mPDF library to generate PDF files, which we updated to version 8.2.0 in a previous ATUM release. This library, in turn, depends on the PSR HTTP Message library. Many other libraries also depend on it, and they may be using outdated versions of this library, which causes the incompatibility.
To find which plugin is causing the incompatibility, you should gradually disable your plugins and try to send the PO again until the issue disappears.
Best Regards,
José Andrés