Out of #373802 (https://bugs.kde.org/show_bug.cgi?id=373802) came an error I noticed in Okular. When I run flatpak run org.kde.okular (with org.kde.okular installed with flatpak-kde-applications) it returns:

Qt: Session management error: Could not open network socket

(process:17434): GLib-GIO-CRITICAL **: g_dbus_message_set_body: assertion '(body == NULL) || g_variant_is_of_type (body, G_VARIANT_TYPE_TUPLE)' failed

(process:17434): GLib-GIO-CRITICAL **: g_dbus_message_set_body: assertion '(body == NULL) || g_variant_is_of_type (body, G_VARIANT_TYPE_TUPLE)' failed
WARNING: bool Phonon::FactoryPrivate::createBackend() phonon backend plugin could not be loaded
WARNING: bool Phonon::FactoryPrivate::createBackend() phonon backend plugin could not be loaded
WARNING: bool Phonon::FactoryPrivate::createBackend() phonon backend plugin could not be loaded
org.kde.knotifications: Audio notification requested, but sound file from notifyrc file was not found, aborting audio notification

it opens a window with the error message:

Unable to find the Okular component: Cannot load library /app/lib64/plugins/okularpart.so: (libpcreposix.so.0: cannot open shared object file: No such file or directory)

now libpcreposix.so.0 does exist on my system in the /lib64 folder. To confirm it was properly installed I ran rpm -qf /lib64/libpcreposix.so.0 and it returned:

pcre-8.39-6.fc25.x86_64

org.kde.okular was installed with (which I'm providing in case it helps to reproduce the error):

git clone https://github.com/KDE/flatpak-kde-applications
cd flatpak-kde-applications
make
flatpak --user remote-add --no-gpg-verify flatpak-kde-applications repo
flatpak --user install flatpak-kde-applications org.kde.okular