I have spun out a testing dashboard site and run a few tests to try and narrow down the issue.
Did the initial pre-test to make sure I still had the same error before doing anything. Yep - internal server error.
Test 1 - Clean WP install - version 5.4 and the beta 2 dashboard (no extensions) and beta child on Divi site running WP 5.5. Worked.
Test 2 - Same as above but updated the default WP themes to the latest versions. Worked.
Test 3 - Updated test dashboard site to 5.5. Worked.
Test 4 - Added in all my preferred extensions. Worked.
Test 5 - Tested old child plugin with test dash. Did not work.
Test 6 - Tested new child with old dash still running WP5.4 and the production version of MainWP. Worked,
There is nothing in the php error log for the child site or the live dashboard.
For the test dash the only php error after all the tests is:
[12-Aug-2020 22:57:25 UTC] PHP Warning: chown(): parameter 2 should be string or int, bool given in /home/*****hidden/public_html/wp-admin/includes/class-wp-filesystem-direct.php on line 196
So I theorised that the problem could potentially be in the production version of the child plugin.
I then tested that theory on a stack of live sites on domains and subdomains and couldn’t create the internal server error for those sites staying with my current dash and child plugin and simply updating to the latest WP 5.5. Don’t you love it when a bug isn’t consistent!
After the 10th hassle free update, I went back to the initial test site and deleted the beta child plugin and put the production one back, and reconnected it to the main dash. Yep - Internal server error again.
Deleted the child and put in the beta child. Worked again.
So now I am stumped.