Sorry if that topic title is confusing. I'm really not sure how else I can describe the issue that succinctly!
Let me clarify with an example:
Say I have three pages: Type 1, Type 2, and Type 3. All pages are using the same template, type-template.
I also have three Page fields: Field A, Field B and Field C. All are dereferenced as multiple page/PageArray, and all are set to choose pages with the above type-template, using checkboxes if that matters. This means that my page choices for Field A are Type 1, Type 2 and Type 3, and the exact same for Fields B and C. (Effectively, these three fields are identical; they're direct copies of one another, in fact.) These fields are currently attached to type-template and no other templates.
Now say that I'm editing Type 1's page, Field A. If I select either or both of Type 2 or Type 3 in Field A, I can save the page and everything works just fine and dandy. But if I select Type 1 as an option in Field A, I get the "Error saving field "field_a" - Page #### is not valid for field_a" error (where #### is the id of Type 1). The same thing happens if I select Type 1 as an option in Fields B or C. No matter which field I'm editing, I cannot select Type 1 as an option while I am editing Type 1...
...but I can select Type 1 and save just fine if I'm editing Type 2's page. Instead, Type 2 becomes "invalid" while I'm editing the Type 2 page. Likewise, Type 3 is "invalid" only while I'm editing Type 3, and perfectly acceptable on the other two Types. I'm not sure whether needing to select the current page as an option is unusual, but Field A et al. define relationships between the three Types, and I often need to say something like "dogs don't like squirrels or cats (Field A), but dogs do like people and other dogs (Field B)". Right now that "dogs like dogs" is impossible to achieve thanks to the error.
I've tried changing the input field type from checkboxes to anything else that lets me choose multiple pages. I've tried all four methods of selectable pages, but I get the error whether I'm selecting using the parent, the template, a custom selector or custom PHP.
I would just shrug and assume that maybe this isn't currently possible, but the really baffling part is that it should be, far as I can tell. This project is very, very similar to an earlier test project, and the other project does not have this issue. If I'm on the "dogs" page there, I can select "dogs" as an option for a field with no problem. I've been back and forth and back and forth comparing the settings of the two fields and the two templates, and I can't see a meaningful difference. Both projects are using the same version of PW (2.5.3), although the functioning one has been steadily upgraded from something like 2.4.10 and this new one was installed as 2.5.3. The only other thing I can think of right now is that the environments are different: the working one is on a remote server and PHP 5.5.7, while the broken one is on a local environment with PHP 5.4.36. I don't know; would that affect this? :/
I've done some searching around and so far have turned up a few threads on similar errors, like https://processwire.com/talk/topic/8400-possible-page-field-bug/ and https://processwire.com/talk/topic/7387-pageautocomplete-page-x-is-not-valid-for-y/ . I've tried to read through them and the threads linked within (keyword: "tried"; it's unreasonably late/early here right now and I'm about to hit the hay after posting this, so maybe another read after I've rested will reveal something!), but most of those seem to have to do with using page ids in custom selectors instead of the page names. I've tried both ids and names and still have this problem, and I've tried just using the parent/template options with similar luck.
While I can move on with other parts of development (and fill in the rest of those fields) for now, this is definitely important for the site I'm building and I'll need to figure something out eventually. Any insight you knowledgable folk can give me—any fixes, any PHP settings that might somehow be the culprit, anything painfully obvious I might've missed at this unfortunate hour—is much appreciated! And let me know if you need me to provide any more information. Thanks!