Very strange. In the first report, tax has clearly been calculated as 18% of your true net sales, not the erroneous net sales of 23,384.90. Had the program calculated tax-exclusive VAT on net sales of 23,384.90, VAT would have been 4,209.28. Had the program mistakenly calculated tax-inclusive VAT on total sales of 26,952.09, net sales would have been reported as 22,840.75. And tax collected would have been 4,111.34. So none of the numbers in the first report match up under any scenario.
The second report looks like what you should see for tax-exclusive line items with net sales of 19,817.71. It seems that by opening the invoices and “saving” them (by which I assume you mean clicking Update), you may have forced the program to rewrite the database entries correctly. Or, there was an error in calculation during generation of the first report. (Reports are not actually saved in Manager. Only their defining parameters are saved. The reports are actually generated from scratch every time you view them.)
If you actually opened every sales invoice and updated it (even without changing anything), I think the first explanation is probably correct. If you only checked some of the sales invoices, you would not have forced overwriting of every one, so the totals would not be correct in the second report. In that case, I think the second explanation is most likely to be correct.