it happens when i try to sign in

Double post, please see https://developers.oxwall.com/forum/topic/34447

Quote from sezar but nothing showing, any idea?

What do you mean with "nothing showing"? The notice you mentioned is not a fatal error.

Does your server support all requirements? Did you perform any changes during backup? As the mentioned error is not a fatal error, it doesn't seem to be the problem.

then how i figure out why i get 500 error, rather how to fix it? where to check for the error?

Please answer our questions.

yes and no for changes

I feel like if this is just a "notice" then this isn't what's causing your blank page(s). I could be wrong about that but notices in my files have never seemed to be reflected on my site itself.

Are you sure you looked in both your error_log at your root and your oxwall_logs > error.log? Are there any other errors or warnings in either of those?

I would start by checking basics if you haven't yet.

Make sure your PHP version has not defaulted to something higher than 5.6. I had this happen to me. For me, the change was really easy to make in my cPanel so it should be something very quick to double-check.

Check your server settings just in case something changed, especially if your host or someone else helped you restore the back-up. Here are the server requirements that you need to follow:
https://developers.oxwall.com/hosting

Make sure cron is set up correctly!:
https://wiki.oxwall.com/install:cron

thank you blaire, people here having the same problems since this software was made, i noticed this through old forum posts, however, i started to think before i grow big, i don't want to waste time and money and headache.

I understand but you also need to look at the dates the software has been updated versus the dates of the postings. A lot of the postings you may bump into are from the older, now outdated versions. 1.8.4 was released late July 2016 so you'll want to pay attention to post dates for comparison. Sometimes they are still relevant and sometimes they have nothing to do with what you're now experiencing.

Also, a lot of times the resolution to those errors are posted. They are typically resolvable.

I've ran Oxwall for over 1 and 1/2 years now and have nearly 13,000 members on my site so I understand pretty well that there is a great learning curve to error troubleshooting, but I don't think it's a poor choice to continue with if someone is willing to put in the dedication (both time and effort). I feel like it's a reasonable trade-off since it's free base software, and that paid software is reducing that inconvenience for a hefty price tag. Personally, I just don't have the money to put upfront for software that needs little to no troubleshooting or error resolution.

There was a time a few months ago that I almost threw in the towel with Oxwall because I was running into SO many errors back to back. A lot of my errors came down to server attacks, weirdly enough, and paying for a content delivery network (CDN) has helped greatly with reducing my stress of potential downtime from the attacks I was experiencing. So, in reality, that had nothing to really do with the Oxwall software and it would've been likely I would've suffered those same problems with other software.
Before then, I had errors with accidentally somehow updating to PHP version 7.1, but I went back down to 5.6 version and haven't had any trouble from that. Oxwall is just not stable with some of the newer PHP versions and you just have to accept that if you want reliability. Sometimes a plugin will cause a 500 error but I have a little more awareness of figuring out which plugin is causing it, and I'm more comfortable going into phpmyadmin and disabling plugins for initial troubleshooting if I need to do it.

But I've stuck with it and with the help of some lovely, lovely people on the support forum here I made it through that rough bump with the software. I'm still learning and still need help too so I certainly can't say I've grown to become an expert with Oxwall. I'm sure I'll need more help in the future when I have other major errors that are beyond my experience with it.

I am also really, really, REALLY happy to be able to buy plugins and support individual developers who are trying to better the software with additional features or capabilities. I intend to buy many more plugins even, but am trying my best to save up for one of the app plugins that a newer developer has created and actively supports.
I, personally, really like the idea of pulling together as a "community" and supporting one another, even if it's just $10 here or there to a nice, solid developer in exchange for a new little visual "upgrade" for my site.

If you do choose to stick with Oxwall then I don't think that's a bad decision at all, but I do understand if you just simply cannot be prepared for such potential troubleshooting and want to make a purchase to reduce that likely possibility.