Well, after executing the snap with strace like this:
$ snap run --strace="-s512 -f" nextcloud-client
It seems that the problem is that it cannot find the QtWebEngineProcess executable inside the snap:
...
[pid 2982] stat("/usr/lib/x86_64-linux-gnu/qt5/libexec/QtWebEngineProcess", 0x7fff396e9ab0) = -1 ENOENT (No such file or directory)
[pid 2982] stat("/snap/nextcloud-client/x1/usr/bin/QtWebEngineProcess", 0x7fff396e9ab0) = -1 ENOENT (No such file or directory)
[pid 2982] rt_sigprocmask(SIG_UNBLOCK, [ABRT], NULL, 8) = 0
[pid 2982] rt_sigprocmask(SIG_BLOCK, ~[RTMIN RT_1], [], 8) = 0
...
Because the path is wrong:
$ find /snap/nextcloud-client/x1/ -name QtWebEngineProcess
/snap/nextcloud-client/x1/usr/lib/x86_64-linux-gnu/qt5/libexec/QtWebEngineProcess
But I’m not really sure what is causing this.
P.D.
Could this also be related?
https://musescore.org/en/node/281322
Because then it’s maybe a problem with the libqt5webenginecore5 package and/or core18!