Hi there. I too have the issue trying to save a document in my User folder. Here are my specs.
Mac Pro 2009 (firmware upgraded to 5,1) - dual X5680 CPUs (12-core 3.33GHz) - 36 GB ram 1333 MHz - Sapphire Pulse RX 590 8GB - Boot drive is a Samsung 860 EVO SSD in a OWC Accelsior S SATA II PCIe card - User accounts are on a Fusion Drive made up of a Samsung 970 EVO Plus 1TB paired with a Western Digital Black Caviar 2TB spinner.
Affinity Designer (now at version 18.3) purchased off Mac App Store (MAS).
Affinity Photo (now at 1.8.3) also purchased off MAS. And I purchased Affinity Publisher direct from your website (not at version 1.8.3).
Can't save anywhere in my User (fusion drive) folder, or on my boot SSD. However, I can save in my User/iCloud/Affinity Designer folder. But that's not where I want to save my files! I have manually reset folder/file permissions, removed then installed all the Affinity apps, cleaned out user, application and system font caches and done a raft of other maintenance scripts using Onyx. NO OTHER applications have issues accessing/saving to my User account folders.
Need help sorting this. Don't know when the issue began because I don't use this lovely software much, but when I do use it I spend days on projects and need it to work. I get the same result whether I use fonts in the document, and/or just vector graphics. I've read it could be too many fonts are activated - I deactivated all User fonts, cleared the caches, restarted and still get the issue. Affinity Publisher has no issues saving into User folder locations, however Affinity Photo has the same issue as Designer. The apps are all very slow to start up, and I see the fontd process is very busy as I wait for an app to start and open up.
What is of concern is that while researching this issue, it can be traced back two years - yet the issue is still occurring for some users and I can find no solution for the problem after more than two hours searching.
Please help - your software is awesome, but this sours the experience considerably for me.