You may see HTML in the product description on the PayPal purchase page, such as this:
Memberships uses the shop's currency settings as a format for the product description on PayPal; many stores have HTML code in the currency settings.
To resolve this, simply remove any HTML code from the currency settings; it should then look something like this:
And finally once that is done, you should enter into the Memberships app, open up each of your created Plans, and re-save them (no need to make any changes before saving).
By saving the plans over again, you will push new information out, and create a product description that does not have HTML in it.
After saving each of your plans, you can then put the HTML back into the currency settings on your shop.
Note: If you need to update the plan again in the future, you will need to follow these steps and remove the HTML from the currency settings again before you update your plan, otherwise it will push an update that puts the HTML back in the description.