This is a known glitch of the old app, and I do recommend using the new beta app.
If for some reason you want to stick with the old app, the glitch you're describing can easily be fixed by changing any setting in the reader top bar.
Change from vertical --> horizontal (or vise versa), change from dark mode to light mode, whatever. Then the text will appear (after you hit "Apply").
Kindle app does not read epub files; limitation imposed by Amazon.
There are various applications that will convert between ebook types. I use Calibre, which is free and quite capable. Just converted two epubs to Kindle mobi files a few minutes ago, and shipped them off to Amazon to display on my Kindles.
For the PC, you can also use any number of readers that will directly access epub files.
Your best bet is to use their email contact as this involves a payment issue email@example.com
But some steps you can take yourself is to contact your card issuer in case it's an issue on their end. Assuming that JNC attempted to charge the card, your card issuer should be able to see what went wrong on their end (if the transaction was flagged as suspicious for example)
I'm in the UK and I seem to recall at least one instance where my bank blocked a payment to JNC and flagged it as suspicious (maybe because it was a foreign currency), so it would be good to get a heads up email.
I'm in similar circumstances. Where I live, USD is a foreign currency that's strictly managed and I could only purchase the subscription using a special dual currency card with certain hard limits. I suspect that time my bank called me isn't going to be the last and there may well be future instances where the payment gets automatically declined.
I went back through my own emails and I've never gotten an email about subscribing, only emails about purchasing extra credits. It would be nice to get some extra notifications, I'm pretty sure I've seen people indicate they'd like a notice about when credits are spent too since there's no real way to track any of that unless you're meticulously noting every time you spend a credit or get a monthly credit
I get an invoice email from my hosting provider (Digitalocean) every month, a similar email from j-novel would be enough for me. That being said, having more granular notifications would also be great!
@qu1nch1a Partly Yes. If it is expired you could either buy it to read it or wait for a catchup month. Every months some series are made free to read for members. Like Black Summoner plus 5 other LN and one Manga this month. So as a member you can read all Volumes of it.