I notice the "Access denied for user s51559" in the SUL Info error message and @jcrespo's "I have blocked the user" at T194343#4200496.

To clarify, the block was needed (and was supposed to be temporary) because not only it was about to break the tool, but all other tools using the same set of databases. Apparently the tools is unmaintained, and we need to find someone to "fix" it so it regain its access again. If you want to help or know someone that can, please direct them to that ticket.

Someone told me there are similar tools available doing similar work but in a better way (but I am sorry, I don't know which ones).

I just read that thread again and they suggest using https://tools.wmflabs.org/guc/ instead, not sure if that would be ok?

jcrespo, I don't know, but see this comment by @Samwalton9:

In T180145#4093787, Samwalton9 wrote:

It turns out we were using the tool because when accounts got merged, CentralAuth sometimes picks the wrong account when given a username. The tool, on the other hand, lists all possible accounts in the case of ambiguity. So while it's a little slower (and a tool rather than on-wiki) it's less likely to cause issues.

The tool is unmaintained and does extremelly expensive DB queries that exhaust the resources, making other tools slow or unusable as @jcrespo mentioned. That said, as I noted on T194343#4202245 I stopped the webservice of the tool after the ban. I'm not sure why it's up again. Thanks.