Salut,
I encountered this issue today… it took me around two hours for tracking down the issue.
It started first with 502 bad gateway, which was indicated by a stopped php-fpm service, which was aborted by a corrupted ini file. (I did a reinstallation of the application of repairing, because of being short on time)
I did a remove & install session of nextcloud ending up with the “appliation can not be installed” message. Tracing down the error is a “Segmentation fault” of the php executable.
After a bit tracking down, I took a core dump, and did a gdb analysis.
The issue is caused by the php-ImageMagik relation
The segfault happens at the end of the PHP process, while cleaning up things:
The snapshot is from executing php oc manually and forcing just a “file not found”:
open("/usr/share/doc/imagemagick-6-common/htmlpolicy.xml", O_RDONLY|O_LARGEFILE) = -1 ENOENT (No such file or directory)
open("/home/nextcloud/.config/ImageMagick/policy.xml", O_RDONLY|O_LARGEFILE) = -1 ENOENT (No such file or directory)
open("/home/nextcloud/.magick/policy.xml", O_RDONLY|O_LARGEFILE) = -1 ENOENT (No such file or directory)
sysinfo({uptime=5407, loads=[27456, 23904, 55264], totalram=2108903424, freeram=1070579712, sharedram=10166272, bufferram=110874624, totalswap=105444
9664, freeswap=1054449664, procs=184, totalhigh=1339031552, freehigh=505876480, mem_unit=1}) = 0
ugetrlimit(RLIMIT_NOFILE, {rlim_cur=1024, rlim_max=1024*1024}) = 0
brk(0x1f26000) = 0x1f26000
brk(0x1f47000) = 0x1f47000
brk(0x1f68000) = 0x1f68000
mmap2(NULL, 233472, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0xb2d2c000
brk(0x1f94000) = 0x1f94000
open("oc", O_RDONLY|O_LARGEFILE) = -1 ENOENT (No such file or directory)
fstat64(1, {st_mode=S_IFCHR|0620, st_rdev=makedev(136, 0), ...}) = 0
write(1, "Could not open input file: oc\n", 30) = 30
munmap(0xb2d82000, 98664) = 0
munmap(0xb2d65000, 115164) = 0
munmap(0xb2df4000, 86232) = 0
munmap(0xb2dd5000, 122896) = 0
munmap(0xb2d9b000, 234676) = 0
munmap(0xb2e0a000, 90328) = 0
munmap(0xb2e21000, 86276) = 0
munmap(0xb2e37000, 86120) = 0
munmap(0xb2e4d000, 77944) = 0
munmap(0xb2e61000, 73836) = 0
munmap(0xb2e74000, 73832) = 0
munmap(0xb2e87000, 73832) = 0
munmap(0xb2e9a000, 118948) = 0
munmap(0xb2eb8000, 98556) = 0
munmap(0xb2ed1000, 73836) = 0
munmap(0xb2ee4000, 336140) = 0
munmap(0xb2fb8000, 82096) = 0
munmap(0xb2f87000, 197396) = 0
munmap(0xb2f62000, 147772) = 0
munmap(0xb2f37000, 173660) = 0
munmap(0xb2fcd000, 82120) = 0
munmap(0xb2fe2000, 245096) = 0
munmap(0xb301e000, 82152) = 0
munmap(0xb3033000, 144548) = 0
munmap(0xb308c000, 90252) = 0
munmap(0xb3057000, 215572) = 0
munmap(0xb3fb4000, 106620) = 0
munmap(0xb31bd000, 102520) = 0
munmap(0xb31d7000, 90228) = 0
munmap(0xb3207000, 362084) = 0
munmap(0xb31ee000, 98736) = 0
--- SIGSEGV {si_signo=SIGSEGV, si_code=SEGV_MAPERR, si_addr=0xc600f8b2} ---
+++ killed by SIGSEGV +++
Here you see the backtrace in the related coredump:
sudo gdb php /tmp/coredump-php.29502
GNU gdb (Debian 7.12-6) 7.12.0.20161007-git
Copyright (C) 2016 Free Software Foundation, Inc.
License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>
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and "show warranty" for details.
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Type "show configuration" for configuration details.
For bug reporting instructions, please see:
<http://www.gnu.org/software/gdb/bugs/>.
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Reading symbols from php...(no debugging symbols found)...done.
warning: core file may not match specified executable file.
[New LWP 29502]
warning: Could not load shared library symbols for 6 libraries, e.g. /usr/lib/php/20151012/xml.so.
Use the "info sharedlibrary" command to see the complete listing.
Do you need "set solib-search-path" or "set sysroot"?
[Thread debugging using libthread_db enabled]
Using host libthread_db library "/lib/arm-linux-gnueabihf/libthread_db.so.1".
bt
Core was generated by `/usr/bin/php oc'.
Program terminated with signal SIGSEGV, Segmentation fault.
#0 0xb356c6e4 in LockSemaphoreInfo () from /usr/lib/arm-linux-gnueabihf/libMagickCore-6.Q16.so.3
(gdb) bt
#0 0xb356c6e4 in LockSemaphoreInfo () from /usr/lib/arm-linux-gnueabihf/libMagickCore-6.Q16.so.3
#1 0xb3718d60 in DestroyWandIds () from /usr/lib/arm-linux-gnueabihf/libMagickWand-6.Q16.so.3
#2 0xb36dd91e in MagickWandTerminus () from /usr/lib/arm-linux-gnueabihf/libMagickWand-6.Q16.so.3
#3 0xb376c284 in ?? ()
Backtrace stopped: previous frame identical to this frame (corrupt stack?)
(gdb)
This is the yunohost-pastebin link: https://paste.yunohost.org/raw/uhusuzabat
This is the diagnostic json: https://paste.yunohost.org/qogeserago
I’ll do a reinstallation of the library at a next step, but If you have further ideas, helps or advice, I would love to know those 
best regards
Matthias