Invalid response Paypal [Solved]

-- HikaShop version -- : 2.6.4

Hi,
I have this message begining the 1 st of october "A paypal notification was refused because the response from the paypal server was invalid" still today! Look on the help section linked with no succes.
It happen few time after the 2.6.4 update and the activation of the product social media icon...
The payment is ok and real on Paypal for all those commands.
I had also this problem during a week end at the begining of september witch solved alone. I expected a Paypal bug because (begining of september) I made no change before and after the problem...
Notice that I use Admin Tools.
Do anyone have this problem this past week end or an idea of my problem ?
Thanks

Thanks Phiipp for this answer,
Yes, as you, I suspect (and hope) just a Paypal server PB...do other people have the same issue this past WE?
This morning from 10h10 the situation came back clean with a completed paypal payment (on one command)!
I had just activated the debug option on paypal plug in and will send it to you if other bugs occurs
Ps : I don't know if there is any relation but I received the the invalid response mails with the spam tag on Windows live mail and not as a spam for the conpleted status...?

Best R

Yes problem came back again (end morning) after only one command ok (mid morning). I didn't made any change on the website!
I followed this https://www.hikashop.com/forum/payment-methods/862282-where-is-payment-log-file-on-hika-business.htmltopic to localize the log file ftp place [media/com_hikashop/upload/safe/logs/report_567395059.log]. Nothing (no log folder) after the last command bug invalid response....but the "safe" folder wasn't in 777 permission so I put it in 777 permisssion and will wait for next bug command (invalid response) to send you the log file !
...
Thanks

Thanks Nicolas for your answer,
Please forget this pb of spam tag, it should have no relation with my Paypal invalid response pb.
This morning on a test command with payment the situation is normal again...but I will like to understand why sometimes (this WE) I have many "Paypal Invald response".
I followed the Debug procedure, probably with a mistake because after this morning command test I can find any log file in [media/com_hikashop/upload/safe/]

There is no "log" folder, only a picture ? and an .htaccess (deny for all)

I had activate the debug optin in the paypal plugin, and allow to my "safe" folder the permission 777. I don't have the log for this morning test command?
www.hikashop.com/forum/payment-methods/8...a-business.htmltopic

That file will only be created if the debug mode of the paypal plugin is turned on and if joomla can create the folder/file in the safe folder. So via FTP, make sure that the safe folder can be written (you can change its permissions to 777).
Then, next time you receive the error email, you should be able to see the debug data in the payment log.

What did I forget ?
Ps edit : today all the command re came with the invalid response again but payment on Paypal OK ! Should it come from a problem of our email, host disponibility...,
Thanks for your help and solutions

I have this on the "payment log file" media/com_hikashop/upload/safe/logs/report_715649334.log
When I click on "see the report" I have a message indicate an empty file but nothing (as see upside) when I search it by ftp?
Do you thing it can be an instability of the plug in or of HS 2.6.4 ?. Should I try to re-install HS? How without losing all my datas (commands, products, config etc...)?
Actually I have around around 20% of my commands who are ok (with confirmed statuts) and around 80 % of my commands with Paypal invald response...?
If I don't find a solution I will try to create an small test website with the same Paypal email to see if the problem come from Paypal or not.

All this is a real pb, hope I will find a solution quickly...On the other forum posts about this "invalid paypal response" I don't found any solution...!
Thanks for your help

Thanks all for you answers,
Pb came from a parameters pb bettweens HS and Admins tools.
I had desactivate Admin Tools and the invalid paypal response went away.
I will have now to find the well parameters for last version of Admin Tools be compatible with HS!
Thanks again.