I also have this problem now as seen in https://bitbucket.org/simon_massey/thinbus-php/issues/6/biginteger-undefined-offset
Have been using the library for a long time on a CentOS server with many different PHP versions including version 7.1 - no problems
Now I moved the code to an Ubuntu 16.04 server with PHP version 7.0.18 and got the problem. I upgraded to version 7.1.11 with the same problem?
I get a lot of warnings like this during login: PHP Notice: Undefined offset: 66 in /var/www/html/lib/srp6a/BigInteger.php on line 3578
It is a trim function.
In the end the script terminates because it exceed the maximum execution time
NB: Also a single line with: Undefined offset: 6
Any suggestions about how to fix this?