[Support request] FAILURE: Error! Failed to insert a new member record. This request will fail.

Hi,
A member tried to subscribe on my client’s website Saturday and this error showed up in the log, but I can’t find any documentation on it.
The Paypal payment worked properly, the IPN was sent and accepted, but the account isn’t created and the payment doesn’t appear in the admin either.
I tried to send the IPN again, and the same thing happened.
Here is the entire log after the IPN was received (I only removed the IP and client email) :

[09/17/2018 1:02 PM] - SUCCESS: Checking if PayPal IPN response is valid
[09/17/2018 1:02 PM] - SUCCESS: Connecting to: https://www.paypal.com/cgi-bin/webscr
[09/17/2018 1:02 PM] - SUCCESS: IPN successfully verified.
[09/17/2018 1:02 PM] - SUCCESS: Creating product Information to send.
[09/17/2018 1:02 PM] - SUCCESS: Transaction Type: Buy Now/Subscribe
[09/17/2018 1:02 PM] - SUCCESS: Item Number: 584
[09/17/2018 1:02 PM] - SUCCESS: Item Name: Paiement Membre associé - groupe
[09/17/2018 1:02 PM] - SUCCESS: Item Quantity: 1
[09/17/2018 1:02 PM] - SUCCESS: Item Total: 150.00
[09/17/2018 1:02 PM] - SUCCESS: Item Currency: CAD
[09/17/2018 1:02 PM] - SUCCESS: Membership payment paid for membership level ID: 5
[09/17/2018 1:02 PM] - SUCCESS: Transaction type: web_accept. Creating member account...
[09/17/2018 1:02 PM] - SUCCESS: swpm_handle_subsc_signup_stand_alone(). Custom value: subsc_ref=5&user_ip=CLIENTIP, Unique reference: 33T750542E094040G
[09/17/2018 1:02 PM] - SUCCESS: Could not find any record using the given email address (CLIENTEMAIL). Attempting to query database using the unique reference: 33T750542E094040G
[09/17/2018 1:02 PM] - SUCCESS: Did not find a match for an existing member profile for the given reference. This must me a new payment from a new member.
[09/17/2018 1:02 PM] - SUCCESS: Creating new member account. Membership level ID: 5
[09/17/2018 1:02 PM] - FAILURE: Error! Failed to insert a new member record. This request will fail.
[09/17/2018 1:02 PM] - SUCCESS: Saving transaction data to the database table.
[09/17/2018 1:02 PM] - SUCCESS: Transaction data saved.
[09/17/2018 1:02 PM] - SUCCESS: Paypal class finished.

It also says “Saving transaction data to the database table” and “Transaction data saved”, but the transaction was NOT actually added to the table.

We recently had to tighten the permissions on our entire WordPress folder. Could this have anything to do with our problem? (Are there folders where this plugin needs to write to be able to access the database?)
Everything worked fine before that.

Thanks for your help.