[KevinH]

What do you mean by "crash" here? A unable to read or write a file should create a "file exception" and Sigil will catch that and at least try to handle it. But an "unknown exception" is not a file exception. That is what is so confusing.

What do you mean by if the css file was already present. You are not trying to read or write directly into Sigil's temp folder are you?

Let's debug from scratch with known good /safe locations. Please try the following and let me know what step causes the failure.

1. On your Desktop create a folder called "test" and copy an html file and its css file into it

2. Fire up Sigil and verify that Sigil is using its default temp folder in Preferences

3. You should have an empty epub after Sigil starts up. Please do a file save as and tell it to write the epub to your Desktop folder to verify it has write permission there.

4. Then use Sigil's Add Existing menu and navigate into the "test" folder on your Desktop created in step 1. Tell it to load the html file you copied there.

Did step 3 pass? Did step 4 pass or fail? Did it give you the same error messages or something different?

I am hoping if this fails it will give us a more informative error message of some sort as this has got to be permissions related in some way.