I used to be able to generate sales tax (GST) reports that showed how much GST I had collected on sales, and how much GST I had paid on purchases. That made it simple to fill in our tax forms and work out how much to pay. The other option on our tax forms is to put in your total sales (including GST) and total purchases, and it would work out the GST components and how much to pay.
I have never been able to work out the later, but used to be able to do the former. Now I cannot do the latter either.
I have two GST accounts and each transaction gets split with the GST components going into the appropriate GST account. When I configure reports, they all show the transaction net value. I cannot seem to generate a report with either just the GST component or the total value of the transaction.
One of the good things about our system is almost everything attracts GST, and there is only one rate.
Anyway - not sure if by brain has developed a bug, or if something has changed in Kmymoney 
I’m really not sure what you mean by “can’t do” either type of reporting. Have options disappeared? Have reports disappeared? Do you get any errors?
What did you previously do? What report did you use, and how was it modified from one of the base reports. I would expect “Transactions by Category” report to do what you want, limiting it to the two desired categories. “Tax transactions by Category” might also work, but I don’t offhand remember the exact difference - it may depend on marking those categories as “Inlcude on Tax reports.”
Note I am assuming that the tax amounts are recorded as a Category, and not coming from or going to a separate account. In this case, you should be able to do a transaction report on just those two accounts.
By can’t do I mean I cannot generate a report that shows either the total value of the transaction or the GST component of the transaction. The reports generated all show the nett value of the transaction - ie without the GST added on. Even if the report only uses the GST accounts which shows the GST portion of the transaction.
Looking again at the documentation, it may be that I have used accounts instead of categories for the GST components. However, I used to be able to generate a report that showed just the GST component of each transaction, now I cannot.
I created a gst-sample.kmy file which shows how to setup the categories and reports.
Two reports are contained: one showing all categories and the other only the GST parts.
Please pay attention to the optional settings of e.g. Purchases and the GST paid which links them and allows to enter the gross amount with the transaction and automatically split the value into net and GST. For this enter the amount and select Purchases as category.
I will take a look at that. I have worked out that if I sort by account, I get the GST components - which is what I must have been doing. Sorting by category doesn’t do this.
Will play about with it a bit more later when I get time.
Cheers
BTW - love the nextcloud