Rise Code block upload stuck 9 out of 10 times

I created a simple ZIP code block file and tried to upload it as code block to empty Rise lesson. Approximately 1 out of 10 times it uploads without a problem, but 9 out of 10 times the same identical file stucks on uploading (upload never finishes, although it is a simple file with only 2,5MB). Does anyone knows what's the catch and what's the workaround for this? The code block is in the attachment.