This is a known issue, which caused by the fact that the declare_compatibility call refers to the support for the High Performance Order Storage feature (HPOS, for short), which is going to be the default in WooCommerce 8.0.
The Aelia Foundation Classes (AFC, for short), which is the framework on which all our plugins are based, supports the HPOS feature and it declares it explicitly, as indicated by the WooCommerce documentation. The AFC is designed to be an independent plugin. It was not bundled with the EU VAT Assistant for years, but downloaded “semi-automatically” upon installation.
A few years ago, the www.remarpro.com “powers” dictated that this was no longer acceptable (even if it worked perfectly) and froze the EU VAT Assistant without notice. They demanded the AFC framework to be bundled with it (i.e. to do the opposite of what we’ve been working on). We found a way to bundle one plugin inside the other (the AFC inside the EU VAT Assistant), and we’ve been doing that, but, apparently, this causes an unexpected issue with the recent WooCommerce versions, in relation to the new High Performance Order Storage feature.
Root cause of the error
Plugins that support the HPOS feature must declare that explicitly. The AFC does just that, but it looks like the declaration of compatibility must be performed within fully independent plugins. In other words, plugins that appear in the WordPress Admin > Plugins list. In short, the AFC must not be an independent plugin, as per www.remarpro.com demands, but it must be an independent plugin if it wishes to declare support for the HPOS feature.
Solution (coming shortly)
This is an edge case that, most likely, wasn’t foreseen by the WooCommerce developers. Removing the declaration for the HPOS support from the framework embedded in the EU VAT Assistant should be sufficient, as this plugin won’t support that feature anyway.
Workaround (also working as a solution)
A workaround to the issue would be to manually install the Aelia Foundation Classes as a standalone plugin. This would disable the framework embedded in the EU VAT Assistant, while preserving all the features.
