Should I have paid stamp duty when buying my ex-husband out of our home?

oldbikebloke

the key to your scenario is the fact you were a joint purchaser with another person

as SDLT geek says, that cancels out the exemption that would have applied had you been the sole purchaser due to a court order (and within the timescale allowed as part of a divorce settlement)

neither of you own a different property so the additional rate SDLT does not apply

the dates you quote means, as geek says, the standard rate applies, hence you (as joint purchasers) had to pay as you personally already owned a property and therefore prevented you as a pair of purchasers from claiming first time buyer relief 

references:
Beware: Tax traps on separation and divorce - Cripps Pemberton Greenish (crippspg.co.uk)

A Court Order
If a couple are considering a judicial form of separation – a divorce, an annulment, a judicial separation, or a separation order – and they agree one or more property transactions in relation to that event, no SDLT will be payable so long as the couple are the only parties to the transactions and the transactions are either

• ordered by the court or
• agreed between the couple before or after the court process, but in contemplation of or in connection with it. As the agreement relates to property it must be a formal written agreement signed by both

Stamp Duty Land Tax: relief for first time buyers - guidance note - GOV.UK (www.gov.uk)

Chapter 3 - Definition of a first time buyer
In order to count as a first time buyer, a purchaser must not, either alone or with others, have previously acquired a major interest in a dwelling or an equivalent interest in land situated anywhere in the world.

I appreciate that is clear as mud, but "major interest" does not mean you own x%, it simply  means: you own a property - full stop. Which is what you already did, you were joint owner with your ex. That fact you were buying an additional share in that property reinforces the fact you already own a property