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 NormalNormal 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