Arch Linux

After a restart the first crash log is the following:
Received signal 11 <unknown> 000000000000
#0 0x7f0ede4aa98e <unknown>
#1 0x7f0ede4a3713 <unknown>
#2 0x7f0ede4aa905 <unknown>
#3 0x7f0ed7a8ee00 <unknown>
#4 0x7f0edde01e8e <unknown>
#5 0x7f0edde0232f <unknown>
#6 0x7f0ede138d2f <unknown>
#7 0x7f0ede14731a <unknown>
#8 0x7f0eddf63e39 <unknown>
#9 0x7f0eddf64001 <unknown>
#10 0x7f0eddf6abe7 <unknown>
#11 0x7f0eddf6ae01 <unknown>
#12 0x7f0eddf4ca51 <unknown>
#13 0x7f0eddf6f6a4 <unknown>
#14 0x7f0eddf6f809 <unknown>
#15 0x7f0ede03e7f5 <unknown>
#16 0x7f0ede41a280 <unknown>
#17 0x7f0ede43da02 <unknown>
#18 0x7f0ede43e64f <unknown>
#19 0x7f0ede43e7d8 <unknown>
#20 0x7f0edc87dcb5 <unknown>
#21 0x7f0ed81eeb1b QObject::event()
#22 0x7f0ed8b8be34 QApplicationPrivate::notify_helper()
#23 0x7f0ed8b93671 QApplication::notify()
#24 0x7f0ed81c38f9 QCoreApplication::notifyInternal2()
#25 0x7f0ed81c6998 QCoreApplicationPrivate::sendPostedEvents()
#26 0x7f0ed8219fd4 <unknown>
#27 0x7f0ed72a33cf g_main_context_dispatch
#28 0x7f0ed72a4f89 <unknown>
#29 0x7f0ed72a4fce g_main_context_iteration
#30 0x7f0ed82195a9 QEventDispatcherGlib::processEvents()
#31 0x7f0ed81c258c QEventLoop::exec()
#32 0x7f0ed81ca896 QCoreApplication::exec()
#33 0x565189335409 main
#34 0x7f0ed7a7b223 __libc_start_main
#35 0x56518933591e _start
r8: 000056518d2f4170 r9: 0000000000000003 r10: 0000000000000000 r11: 00007f0ed7be4020
r12: 00007ffcaa0e0920 r13: 00007ffcaa0e0940 r14: 0000000000000001 r15: 000056518a75eb50
di: 000056518a75eb50 si: 000056518d2f4170 bp: 00007ffcaa0e08f0 bx: 000056518e8e0a90
dx: 000056518a75eb60 ax: 000056518e8b8870 cx: 000056518a75eb60 sp: 00007ffcaa0e08b0
ip: 00007f0edde01e8e efl: 0000000000010202 cgf: 002b000000000033 erf: 0000000000000000
trp: 000000000000000d msk: 0000000000000000 cr2: 0000000000000000
[end of stack trace]
Calling _exit(1). Core file will not be generated.

Downgrading qt5-webengine from version 5.12.0-1 => 5.11.2-2 temporarily solves the issue.

`nextcloudcmd --version` gives:

Using Qt 5.12.0, built against Qt 5.11.2

And indeed downgrading qt5-webengine to the "built against" version solves the issue.

Note that after downgrading, `nextcloudcmd --version` still shows "Using Qt 5.12.0" but the problem is solved nonetheless.

Since Qt 5.12.0 is the version currently distributed and a few days ago a path was added to enable compatibility with 5.12, I thought maybe a simple rebuild on a system running Qt 5.12 would do the job. The build process itself runs flawlessly and `nextcloudcmd --version` gives:
Nextcloud version 2.5.1git
Git revision b37cbea5cc12b01d9baa0e36a996c873acdeea00
Using Qt 5.12.0, built against Qt 5.12.0
Using 'OpenSSL 1.1.1a 20 Nov 2018'

However when entering the credentials, it still exits with:

Thread 1 "nextcloud" received signal SIGSEGV, Segmentation fault.
0x00007fff700f52f0 in ?? ()
(gdb) backtrace
#0 0x00007fff700f52f0 in ()
#1 0x00007ffff2d78e91 in () at /usr/lib/libQt5WebEngineCore.so.5
#2 0x00007ffff2d7932f in () at /usr/lib/libQt5WebEngineCore.so.5
#3 0x00007ffff30afd2f in () at /usr/lib/libQt5WebEngineCore.so.5
#4 0x00007ffff30be31a in () at /usr/lib/libQt5WebEngineCore.so.5
#5 0x00007ffff2edae39 in () at /usr/lib/libQt5WebEngineCore.so.5
#6 0x00007ffff2edb001 in () at /usr/lib/libQt5WebEngineCore.so.5
#7 0x00007ffff2ee1be7 in () at /usr/lib/libQt5WebEngineCore.so.5
#8 0x00007ffff2ee1e01 in () at /usr/lib/libQt5WebEngineCore.so.5
#9 0x00007ffff2ec3a51 in () at /usr/lib/libQt5WebEngineCore.so.5
#10 0x00007ffff2ee66a4 in () at /usr/lib/libQt5WebEngineCore.so.5
#11 0x00007ffff2ee6809 in () at /usr/lib/libQt5WebEngineCore.so.5
#12 0x00007ffff2fb57f5 in () at /usr/lib/libQt5WebEngineCore.so.5
#13 0x00007ffff3391280 in () at /usr/lib/libQt5WebEngineCore.so.5
#14 0x00007ffff33b4a02 in () at /usr/lib/libQt5WebEngineCore.so.5
#15 0x00007ffff33b564f in () at /usr/lib/libQt5WebEngineCore.so.5
#16 0x00007ffff33b57d8 in () at /usr/lib/libQt5WebEngineCore.so.5
#17 0x00007ffff17f4cb5 in () at /usr/lib/libQt5WebEngineCore.so.5
#18 0x00007fffed165b1b in QObject::event(QEvent*) () at /usr/lib/libQt5Core.so.5
#19 0x00007fffedb02e34 in QApplicationPrivate::notify_helper(QObject*, QEvent*) () at /usr/lib/libQt5Widgets.so.5
#20 0x00007fffedb0a671 in QApplication::notify(QObject*, QEvent*) () at /usr/lib/libQt5Widgets.so.5
#21 0x00007fffed13a8f9 in QCoreApplication::notifyInternal2(QObject*, QEvent*) () at /usr/lib/libQt5Core.so.5
#22 0x00007fffed13d998 in QCoreApplicationPrivate::sendPostedEvents(QObject*, int, QThreadData*) () at /usr/lib/libQt5Core.so.5
#23 0x00007fffed190fd4 in () at /usr/lib/libQt5Core.so.5
#24 0x00007fffec21a3cf in g_main_context_dispatch () at /usr/lib/libglib-2.0.so.0
#25 0x00007fffec21bf89 in () at /usr/lib/libglib-2.0.so.0
#26 0x00007fffec21bfce in g_main_context_iteration () at /usr/lib/libglib-2.0.so.0
#27 0x00007fffed1905a9 in QEventDispatcherGlib::processEvents(QFlags<QEventLoop::ProcessEventsFlag>) () at /usr/lib/libQt5Core.so.5
#28 0x00007fffed13958c in QEventLoop::exec(QFlags<QEventLoop::ProcessEventsFlag>) () at /usr/lib/libQt5Core.so.5
#29 0x00007fffed141896 in QCoreApplication::exec() () at /usr/lib/libQt5Core.so.5
#30 0x00005555555fd479 in main ()

It occurs to me, that the error does not lie in the version mismatch but rather in the new version of qt5-webengine.
Thus, downgrading qt5-webengine did the job, thanks for the helpful comments.

I managed to add a patch to nextcloud-client and get rid of crashes. The patched version 2.5.1-2 can be found from [community-testing]. Please test it, thanks!

Ok. I have managed to test it, and can confirm that it doesn't crash for me.
Note that the package is already in [community] and not in [community-testing].

Yep! I got several positive testing results, so I've pushed it to [community] a few hours ago. Thank you for the feedback, too!