This is likely to be an issue with memory, bulk export requires a lot of memory to create the PDF. You can verify this by looking at the PHP error that triggers the 500 error. More information here: 500 Internal Server Error / blank page.
If you can export single invoices fine, you could try to increase the number of invoices selected to see what your limit is.
You can either increase the memory available for WordPress/PHP on your server, or alternatively you may want to look at the Professional extension, which has a bulk export feature that doesn’t create a single big PDF file that holds all the invoices (which consumes a lot of memory) but creates separate PDF files for each invoice and then lets you download it in a zip.
Hope that helps!
Ewout
