UPDATE: The #REF! errors were not returned randomly in a broad sense, they appear sometimes and once the first occurrence appear on one cell then it repeats to the cells below it.

This looks to be a latency problem inherent to GOOGLEFINANCE(), as was previously mentioned by Normal in a comment to the question.

In order to have a "rock-solid formula" instead of using GOOGLEFINANCE directly in column E use an auxiliary sheet to hold the historical currency conversion rates as fixed values instead of calculated values through a formula.

You could use GOOGLEFINANCE() to get that data but then copy and paste values only.

Short answer before the OP shared a demo spreadsheet

Instead of using ambiguous date format (00/00/0000) use a explicit one like

• dd/mmm/yyyy
• mmm/dd/yyyy

among others

Explanation

The problem could be due to the regional settings / date format of your spreadsheet / data.

I just tried to reproduce the problem but it works fine in my environment:

• Date format: dd/mm/yyyy
• Regional settings: Mexico
• Account language: Spanish Latin America
• Always show the function names in English

Reference

• GOOGLEFINANCE - Google Docs editors Help

Instead of getFormulas and setFormulas, use copyTo method:

range.copyTo(newRange);

This will have the same effect as if you selected the last row and copied it below. In particular, it will also copy down any values that were entered directly into cells. If you prefer such values to not be copied, use getFormulas/setFormulas combination on newRange:

range.copyTo(newRange);
newRange.setFormulas(newRange.getFormulas());

The effect of the last line: no change of formulas, but the cells that didn't contain a formula will become empty.

R1C1 approach

You can also try replacing the last three rows of addRow with

var formulas = range.getFormulasR1C1();
var newRange = sh.getRange(lRow+1,1,1,lCol);
newRange.setFormulasR1C1(formulas);

This will make sure the references in formulas are remapped as if you copy-pasted them. However, this has a major limitation: setFormulasR1C1 doesn't work correctly with empty string as its argument, which will result in #ERROR! messages in the cells that should be left blank.

The reason R1C1 works differently is that getFormulasR1C1 retrieves formulas in a relative form: for example, a reference from A3 to D2 becomes R[-1]C[3], meaning "one row up, three columns to the right". When the formula is set to, for example, E4 with setFormulasR1C1, the cell reference will appear as H3.