Hi Oceanfish26,
Can you please check the following if below number sequences are set to 'Continuous' and what is the next available number is set.
Go to Organization administration > Number sequences.
-- Select "Travel and expense" in the 'Area' section and then select Expense report number.
-- Similarly in the same form, just select Expense transaction number and check that too.

You can also check by clearing the lines available in "Status list" of these number sequences.
You can clear by using Alt + F9.

Regards,
Sourav