when uploading files from my Mac (less than 2GB) directly to Galaxy they never finish uploading. They get stuck at around 58% and then it gives me an error message (picture attached). Is there something I can do to fix this?
What Galaxy is this? The error suggests that the upload is just too slow and times out, which could be problem at any point between you and the Galaxy you are uploading too.
To add to what @marten states, there are alternative ways to upload data to Galaxy.
FTP is a particularly good choice for larger files and slower connections. It looks like where you are working has FTP enabled. FTP can be done line-command or with a client.
See these FAQs for details: https://galaxyproject.org/support/