I have no clue, it seems like somethin is not synced????
# LC_ALL=C pacman -Syu
:: Synchronizing package databases...
 obcore is up to date
 obextra is up to date
 obcommunity is up to date
 obmultilib-testing is up to date
 obmultilib is up to date
 observice is up to date
 core is up to date
 extra is up to date
 community-testing is up to date
 community is up to date
:: Starting full system upgrade...
resolving dependencies...
looking for conflicting packages...

Packages (12) 66-0.2.5.2-1  at-spi2-atk-2.34.2-1  glib2-2.64.0-1
              haskell-sandi-0.5-90  hedgewars-1.0.0-43  imagemagick-7.0.9.27-1
              libwbclient-4.10.8-3  linux-lts419-4.19.107-2  oblibs-0.0.5.1-1
              re2-1:20200101-1  smbclient-4.10.8-3  wayland-protocols-1.20-1

Total Download Size:   149.05 MiB
Total Installed Size:  302.08 MiB
Net Upgrade Size:        0.01 MiB

:: Proceed with installation? [Y/n] y
:: Retrieving packages...
error: failed retrieving file 'haskell-sandi-0.5-90-x86_64.pkg.tar.zst' from mirrors.evowise.com : The requested URL returned error: 404
error: failed retrieving file 'haskell-sandi-0.5-90-x86_64.pkg.tar.zst' from mirror.rackspace.com : The requested URL returned error: 404
error: failed retrieving file 'haskell-sandi-0.5-90-x86_64.pkg.tar.zst' from ftp.hosteurope.de : The requested URL returned error: 404
error: failed retrieving file 'haskell-sandi-0.5-90-x86_64.pkg.tar.zst' from mirror.fra10.de.leaseweb.net : The requested URL returned error: 404
error: failed retrieving file 'haskell-sandi-0.5-90-x86_64.pkg.tar.zst' from mirror.metalgamer.eu : The requested URL returned error: 404
error: failed retrieving file 'haskell-sandi-0.5-90-x86_64.pkg.tar.zst' from mirror.metalgamer.eu : The requested URL returned error: 404
error: failed retrieving file 'haskell-sandi-0.5-90-x86_64.pkg.tar.zst' from mirror.mikrogravitation.org : The requested URL returned error: 404
error: failed retrieving file 'haskell-sandi-0.5-90-x86_64.pkg.tar.zst' from mirror.mikrogravitation.org : The requested URL returned error: 404
^Cerror: failed retrieving file 'haskell-sandi-0.5-90-x86_64.pkg.tar.zst' from ger.mirror.pkgbuild.com : The requested URL returned error: 404

Interrupt signal received
I checked on arch linux and there is 0.5-91 already available, 0.5-92 is staging.. So not sure why...
You need to sync your mirrors # pacman -Syu again
community/haskell-sandi 0.5-91 0.45 MiB 0.07 MiB

It seems that you were trying to get the older version meanwhile they replaced it with a newer version
Are you sure... 
# LC_ALL=C pacman -S haskell-sandi
resolving dependencies...
looking for conflicting packages...

Packages (1) haskell-sandi-0.5-91

Total Download Size:   0.07 MiB
Total Installed Size:  0.45 MiB
Net Upgrade Size:      0.00 MiB

:: Proceed with installation? [Y/n] y
:: Retrieving packages...
error: failed retrieving file 'haskell-sandi-0.5-91-x86_64.pkg.tar.zst' from ftp.icm.edu.pl : The requested URL returned error: 404
error: failed retrieving file 'haskell-sandi-0.5-91-x86_64.pkg.tar.zst' from ftp.icm.edu.pl : The requested URL returned error: 404
error: failed retrieving file 'haskell-sandi-0.5-91-x86_64.pkg.tar.zst' from arch.midov.pl : The requested URL returned error: 404
error: failed retrieving file 'haskell-sandi-0.5-91-x86_64.pkg.tar.zst' from arch.nixlab.pl : The requested URL returned error: 404
error: failed retrieving file 'haskell-sandi-0.5-91-x86_64.pkg.tar.zst' from arch.nixlab.pl : The requested URL returned error: 404
error: failed retrieving file 'haskell-sandi-0.5-91-x86_64.pkg.tar.zst' from mirror.onet.pl : The requested URL returned error: 404
error: failed retrieving file 'haskell-sandi-0.5-91-x86_64.pkg.tar.zst' from piotrkosoft.net : The requested URL returned error: 404
warning: failed to retrieve some files
error: failed to commit transaction (failed to retrieve some files)
Errors occurred, no packages were upgraded.
I should say that i had similar problem while installing obarun... I had polish mirrors on in mirrorlist. Imagemagick was problematic to download, it had the same error 404... I changed mirrors to worldwide and imagemagick downloaded.
But with this haskell sandi even downloading from worldwide fails.
# LC_ALL=C pacman -Sy haskell-sandi
:: Synchronizing package databases...
 obcore is up to date
 obextra is up to date
 obcommunity is up to date
 obmultilib-testing is up to date
 obmultilib is up to date
 observice is up to date
 core is up to date
 extra is up to date
 community-testing is up to date
 community is up to date
resolving dependencies...
looking for conflicting packages...

Packages (1) haskell-sandi-0.5-91

Total Download Size:   0.07 MiB
Total Installed Size:  0.45 MiB
Net Upgrade Size:      0.00 MiB

:: Proceed with installation? [Y/n] y
:: Retrieving packages...
error: failed retrieving file 'haskell-sandi-0.5-91-x86_64.pkg.tar.zst' from mirrors.evowise.com : The requested URL returned error: 404
error: failed retrieving file 'haskell-sandi-0.5-91-x86_64.pkg.tar.zst' from mirror.rackspace.com : The requested URL returned error: 404
error: failed retrieving file 'haskell-sandi-0.5-91-x86_64.pkg.tar.zst' from mirror.rackspace.com : The requested URL returned error: 404
warning: failed to retrieve some files
error: failed to commit transaction (failed to retrieve some files)
Errors occurred, no packages were upgraded.
Fungalnet is right.
look at the version: https://www.archlinux.org/packages/community/x86_64/haskell-sandi/
force the synchronization: 
# pacman -Syy haskell-sandi
if it fail again, try (like you did) different mirrors. Your issue is clearly depending of the mirror and the synchronization of your local database
Arch is like quick sand, from 90 went to 91 now to 92 
cd /var/cache/pacman/pkg
sudo wget https://archimonde.ts.si/archlinux/community/os/x86_64/haskell-sandi-0.5-92-x86_64.pkg.tar.zst
sudo wget https://archimonde.ts.si/archlinux/community/os/x86_64/haskell-sandi-0.5-92-x86_64.pkg.tar.zst.sig
sudo pacman -U haskell-sandi-0.5-92-x86_64.pkg.tar.zst
or 
sudo pacman -S haskell-sandi if you use that mirror
will work till it gets to the next version
That's a good mirror, it seems to be ahead of mine as well. Look down the list for Slovenia

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