By now I've almost finished installing Shadowrun Hong Kong with GOG Galaxy, but throughout the process I repeatedly got the mentioned error.
Is that just something that sometimes happens with Galaxy (beta is beta and so on), or is there something I can do to reduce the risk of it happening?

Anyone knows?

To reduce it to zero just don't use Galaxy. :)

It could be your anti-virus which is blocking something. Disable it and see if you can install a game without problems. Or try to run Galaxy as administrator and see if this helps.

I should have mentioned this in my first post, but I have installed several games with no issues. I did experience the same bug a couple of times with Witcher 3, but nowhere near as bad as with Shadowrun: Hong Kong.

I did consider just downloading it with the browser. But I do kind of like a client listing all my installed GOG games :)

Most probably is your anti virus blocking acces to a certain file. Personally I've added my whole game partition (including GOG downloads folder) to antivirus real-time exception list.

It happens when there is checksum in a game. It is nothing to worry about.
Btw at a glance I read your problem as 'dick asses error'. :p
sorry about that.

You can import your games later to Galaxy.

== IMPORT ALREADY INSTALLED GAMES TO GALAXY ==

For GOG games installed using installers from the last few months (so called Galaxy-compatible installers)
Click the "+" button on top of the sidebar and select "scan and import folders" - it will find all compatible games within that folder and add them to the Client.

https://www.gog.com/forum/general/gog_galaxy_beta_2/post1

Thanks for the various tips guys. I'll try them out if/when I encounter the problem again :)

I turned off the antivirus while downloading and it worked