I have an issue similar to these ( Problems with lv2 plugins on Ardour 7.3 and Calf plugins - Failed to instantiate ) – though unlike the calf-specific post it seems to apply to all LV2 plugins. I just wanted to report, because it wasn’t mentioned in previous posts, this seems to affect only newly inserted LV2 plugins, not existing LV2 plugins (even when loading a session from disk that has LV2 plugins, they work fine).
Manjaro, fully updated as of today, Ardour 7.5.0 binary from this site, plugins are from Manjaro repo or AUR. Also, loading plugins in something like Carla seems to work (though I have not actually tried to use them there), seems like an issue specific to Ardour (or a library version mismatch as stated before).